- Email: [email protected]
Interaction between sustained and transient channels: Form inhibits motion in the human visual system
INTERACTION BETWEEN SUSTAINED AND TRANSIENT CHANNELS: FORM INHIBITS MOTION IN THE HUMAN VISUAL SYSTEM’ MICHAEL
Max-Planck-Institut
W. VON GRLWAU
fiir Psychiatric. Kraepelinstr. 2. 8 Miinchen 40. Germany
(Rrcrired 16 Febrwry
1977: in revisedform 25 &fay 1977)
Abstract-The effect of changing spatial frequency content of the inducing flashes on the likelihood of seeing apparent motion was investigated The results suggest that the sustained channel inhibits the transient channel. This inhibitory effect is largest in the fovea and decreases sharply towards the periphery. Conditions that weaken the sustained channel lead to a decrease of the inhibitory effect and a corresponding increase in the response of the transient channel.
INTRODL’CY’IO’ION
Much psychophysi~l and neurophy~olo~~l evidence has been accumulated over the past ten years. demonstrating the existence of two parallel processing
systems in the visual system, the transient and sustained channels (e.g. Enroth-Cugell and Robson, 1966; Cleland. Dubin and Levi&, 1971; CIeland, Levick and Sanderson, 1973; Ike& and Wright, 1974; 1975; Kulikowski and Tolhurst 1973; Tolhurst. 1973). The transient channel is characterized by high temporal resolution (responding optimally to fast movement, flicker, abrupt stimulus on- and offsets etc.). but low spatial resolution (mainly concerned with low spatial frequencies). The sustained channel, on the other hand, shows low temporal resolution (responding optimally to slow movement, stationary targets, prolonged presentations, etc.), but high spatial resoiution (preferring high spatial frequencies) The transient channel therefore is thought to process information about stimulus location and change of location or motion, while the sustained channel processes information about the spatial detail or form of the stimulus. The two systems are not inde~dent, however. Singer and Bedworth (1973) have demonstrated that the sustained channel is inhibited by the transient channel, and psychophysical experiments on apparent motion implicate reciprocity in these interactions. since. on the one hand, motion isolation inhibits form information (Breitmeyer, Love and Wepman. 1974; Breitmeyer, Battaglia and Weber, 1976) and, on the other hand, form information has been shown to affect both the color of the apparently moving object (Kolers and von Griinau, 1976) as well as the likdihood of perceived motion (Orlansky, 1940; Frisby, 1972; Kolers, 1972). The way in which the sustained channel influences the transient channel has not yet been clarified, however, neither neurophysiologically nor psychophysically. In the present report the interaction between the two channels in apparent motion is studied by mani’ This work was partially supported by funds from the Deutsche Forschungsgemeinschaft SFB 50, Kybemetik. 197
pulating the relative stimulus effectiveness in exciting the two channels. This is done by changing the spatial frequency content and the retinal position of the inducing flashes. METHODS Appararus
A schematic representation of the 3-channel Maxwellian View system which was used for the presentation of the stimuli is shown in Fin. la. The linht sources consisted of IO0 W Halogen lamp<. The shutteis were EEG magnets whose timing was controlled by electronic circuitry. Intensity of the fight beams was regulated by insertion of neutral density filters. The observer viewed the display with his right eye through an exit pupil of 2 mm dia. A small additional 4-D lens could be inserted in front of it. The effect of this lens was to defocus the stimuli. i.e. to reduce their high spatial frequency content. The observer’s head was stabitized by in~vid~lly fitted bite bars, The stimuli were dark 0.5’ squares on a light (1.51~~) background field of 7” dia. Thev aooeared one at a time 1.4” above the horizontal midline df’the display (Fig. I& Their separation(S) was I”, 2” and 4’. A dark fixation point was presented halfway between the two stimuli, either on the midline (0’) or 2” below. The stimuli were flashed for 100 msec each with a pause between them (ISI) of either 0, 25. 50, 75 or IOOmsec. The interval between the offset of the second and the following onset of the first stimulus (ICI) was set at l..Ssec, so that apparent motion occurred in the left-to-right direction only. Procedure The likelihood of seeing motion was measured for 3 different separations of the inducing flashes with central fixation. The inducing flashes thus came to lie in the foveai region (S = I”), the parafovea (S = 2”) and the near periphery (S = 4’). while the motion path always had to cross the fovea. Likelihood of motion was also measured at a separation of 2’ when the fixation point was 2” below the motion path, so that the inducing flashes as well as the motion path came lo lie extrafoveally. All these measurements were done both with and without the defocusing lens. Within these 8 conditions 40 presentations were given at each of 5 values of ISI. Conditions and values of IS1 appeared in a predetermined randomized order. Before every session the observer was dark-adapted for about 20min and then light-adapted for a few minutes
198
Fig. 1. (a) Schematic representation of the 3-channel Maxwellian View system. (b) Stimulus display. Only one flash was present at any one time. S = separation, FP = ftxation point. to the average luminance level of the display. After neglecting the first 3 responses an average of 5 responses were recorded at one ISI before switching to the next. After every presentation (during the ICI) the observer responded with a “yes” or “no” to the question: Do you see smooth continuous motion of one object across the whole distance?
The two observers had good color and spatial vision as tested with the H-R-R Pseudo-isochromatic plates and the Bausch & Lomb Orthorater. They were both well-prac-
ticed in psychophysical experiments. and LW was completely unaware of the purpose of this study. RESULTS
Analyses of variance were performed on the results separately for the two observers. They yielded highly significant effects at the 0.01 level for all variables and interactions, except that for observer LW the Iqns effect is significant at the 0.05 level and the lens-
C
./’
[ t
..Y
.
I 0
is
50
7s
1
i 100
ISI
0
2s
SO
7s
Cmsl
100
0 Fmttmn
2
01 FP [“I
Fig. 2. Likelihood of seeing apparent motion as a function of ISI for two observers. when fixation is in the center (A, D) or in the periphery (B, E). C and F: Overall effect of fixation point. o--_-o without lens, O--+
with lens.
Interaction between sustained and transient channels
separation interaction is not significant. Because of the significance of the triple interaction, further tests on the simple 2-way interactions were done. Relevant simple main effects were also tested. Plotting likelihood of motion as a function of ISI leads in all cases to the characteristic inverted-U relationship. Figure 2 shows some curves of this kind with absence or presence of the defocusing lens as parameter. For both observers the left graph (A and D) is for a separation of 2” at the central fixation point, while the middle graph (B and E) is for the same separation, but for a fixation point 2” below the motion path. The interactions in A and D are both highly significant (P < 0.01). while the interactions in B and E are not significant. This means that in A and D the curves for the condition with the lens are shifted toward shorter ISI, while in B and E they are essentially parallel. Aside from the shift toward shorter ISI, introduction of the defocusing lens also leads to a significant (P < 0.05 for LW. P < 0.01 for MG) overall increase in the likelihood of seeing motion, but only when fixation is central and not when it is peripheral (C and F). Defocusing also has differential effects depending on the separation between the two inducing flashes: The effect of the lens (shift toward shorter IS1 and increase in likelihood of motion) is large for the two small separations, when the inducing flashes lie in fovea1 and parafoveal regions. but almost disappears at the large separation. This is graphed in Fig 3 for both observers. Moving the fixation point from the center to 2” below the midline, has an effect similar to defocusing. Without the lens, moving to the periphery shifts the likelihood-of-motion vs IS1 relationship toward shorter ISI and increases the likelihood of seeing motion. With the lens there is only a small increase in the likelihood of motion and no shift (Fig. 4).
10
LW
199
The separation between the two inducing flashes influences the likelihood of seeing motion only insofar as the smallest separation, i.e. when both flashes lie in the fovea, leads to significantly more reports of apparent motion for both observers than the other two separations (see Fig 3). DISCUSSION The effect of the extra lens was to defocus the inducing flashes. In terms of spatial frequency content this means that high frequencies were largely removed from the spectrum, while low frequencies were hardly affected. The effective stimulus situation for the transient channel therefore was not changed and one would not expect any changes in its response. i.e. the likelihood of apparent motion or its temporal parameters should remain the same. The effective stimulus situation for the sustained channel, however, was changed drastically. One would expect that its response is reduced considerably, including inhibitory effects that it might have onto the transient channel. This would have the effect of weakening the processing of form information, while it would simultaneously strengthen the processing of motion information. This again would result in impaired form perception and improved perception of motion. That defocusing has differential effects on the transient and sustained channels was shown by Ikeda and Wright (1972) While the responsiveness of sustained cells was reduced considerably even by small amounts of blurring, the responsiveness of transient cells was relatively unaffected even by well-defocused stimulus images. Thus a 4-D defocusing lens was found to affect both channels in a drastically different way. Form perception was not tested here, but the present experiment demonstrated quite clearly that likelihood of seeing motion is increased when the in-
A
0
Fig. 3. Likelihood of seeing apparent motion as a function of ISI for two observers, when separation between the two inducing flashes is 1” (A, D), 2” (B, E) or 4’ (C. F). W without lens, M with lens.
MICHAEL W. LO? GR~L.AL
ISI [ml
Fig. 4. Likelihood of seeing apparent motion as a function of ISI for two observers without (A, C) and with (B. D) the defocusing lens. O--O fixation in the periphery, +---o fixation in the center.
ducing flashes are defocused. Furthermore, motion occurs at shorter intervals between the two inducing flashes, i.e. processing is done about 25 msec faster. This corresponds well with the temporal differences between the transient and sustained channels as reported in the physiological literature (Dow, 1974; Fukuda, 1973; Ikeda and Wright, 1975) and might reflect a greater prominence of the transient channel with its shorter latencies. For a gross perceptual event like apparent motion one must assume that a certain amount of averaging between short- and long-latency responses takes place. Thus motion occurs over a large range of ISI, and a shift of this distribution towards shorter ISI means that the relative importance of short-latency responses is increased Further analysis, however, shows that defocusing has its effects only when fixation is in the center. With peripheral fixation both effects disappear. This finds its explanation in the fact that the ratio of transient to sustained units is somewhat larger in the periphery than it is in the central fovea (Wiissle, Levick and Cleland, 1975; Fukuda and Stone, 1974). The inhibitory effect of the sustained channel onto the transient channel therefore will be weaker in the periphery. This, of course, leads to an increase in the likelihood of motion in this area (see Kolers and von Griinau, 1977, and the present Fig 4). Defocusing of the inducing flashes here has no additional effect, since the sustained channel is already of much less importance. The reciprocity between changing the frequency content or changing the retinal position of the inducing flashes also becomes clear from Fig 4. When the flashes are focused (A and C), changing fixation point from center to periphery brings an increase in motion as well as a shift to shorter ISI. When the &&es are defocused, however, changing fixation point has only a slight effect. It follows that changing fixation
point, i.e. moving into a peripheral, less form-sensitive area, can have an effect only when the sustained channel is not already weakened by e.g. defocused stimuli. Leaving the fixation point in the center, but moving the inducing flashes into the periphery by changing their separation leads to effects that are not quite as straightforward. On the one hand, the effects of the defocusing lens almost disappear at the large separation, in line with the above argument On the other hand, apparent motion is seen most often with the smallest separation, when the inducing flashes he in the fovea, but still less than with peripheral fixation. Two more factors have to be considered here. First. separation between the inducing flashes as such has been shown to all&t the system’s ability to create apparent motion (Wertheimer, 1912; Korte. 1915) Thus motion is generally seen more readily when the flashes are closer together. Secondly, it was observed in Kolers and von Griinau (1977) that the likelihood of motion is decreased when motion has to go through the fovea, even if the inducing stimuli he well in the periphery. The retinal position not only of the inducing gashes but also of the motion path seems to determine whether motion is seen or not. In the present optical system the extra lens leads to a slight reduction of image size. This effect is ap proximately compensated by the enlargement due to defocusing. Overall this results in stimulus images of roughly the same area with, however, slightly reduced contrast. According to Korte’s Law (Korte, 1915) this should give a slight increase of optimal ISI. working against the eflbct observed here. that optimal ISI shifted towards lower values with the extra lens In summary, the results of the present experiment suggest that the sustained channel inhibits the transient channel. This inhibitory ef?&t is largest in the fovea and decreases sharply towards the periphery. Conditions that weaken the sustained channel (like
201
interaction between sustained and transient channels defocusing of the stimuli) lead to a decrease of the inhibitory effect and a corresponding increase in the response of the transient channel. It appears therefore that the interactions between
the two channels are symmetrical, each channel inhibiting the other. Both interactions also seem to occur concurrently since during apparent motion the second inducing flash has been shown to have a metacontrast effect on the first fiash (Breitmeyer, Battaglia and Weber, 1976). This kind of masking effect has been linked by Breitmeyer and Ganz (1976) to an inhibition of the first flash’s sustained response by. the second flash’s transient response. A similar inhrbrtory effect of the first flash’s sustained response onto the second gash’s transient response is suggested by the present results. Direct physiological evidence is not available as yet, but strong indications for the existence of such an interaction have been observed (Singer, personat
co~uni~tion~
Enroth-Cugell C. and Robson J. G. (1966) The contrast sensitivity of retinal ganglion cells of the cat. J. Physiol. 187. 5 17-552. Frisby J. P. (1972) The effect of stimulus orientation on the phi phenomenon. Vision Res 12, 1145-l 166. Fukuda Y. (1973) Differentiation of principal cells of the rat lateral geniculate body into two groups: fast and slow cells. Exul Brain Res. 17. 242-260. Fukuda Y. and’ Stone J. (1974) Retinal distribution and central projections of Y-. X- and W-cells of the cat’s retina J. Ntwrouhvsioi. 37. 749-772. Ikeda H. and Wright M. J. (1972) Differential effects of refractive errors and receptive field organization of central and peripheral ganglion cells. Vision Res. 12, 1465-1476. Ikeda H. and Wright M. J. (1974) Evidence for “sustained” and “transient” neurones in the cat’s visual cortex. Vision Res. 14. 133-136. Ikeda H. and Wright M. J. (1975) Spatial and temporal properties of “sustained” and “transient” neurones in area 17 of the cat’s visual cortex. Expl Brain Res. 22. 363-383.
Acknowfedgemmrs-I wish to thank Dr. W. Singer for the opportunity to conduct this research and for his valuable advice, Dip.-Ing. G. Neumann for technical assistance with the electronics and LW for being a patient observer. REFERENCES
Breitmeyer B. G. and Ganz L. (1976) Implications of sustained and transient channels for theories of visual pattern masking. saccadic suppression, and information processing. Psychof. Rea 83, l-36. Breitmeyer B. G., Love R. and Wepman B. (1974) Contour suppression during stroboscopic motion and metacontrast. Yisiott Res. 14, 1451-1456. Breitmeyer B. G., Battaglia F. and Weber C. (1976) U-shaped backward contour masking during stroboscopic motion. J. exp. P&ol. Human Perception and Performawe 2. 167-I 73. Cleland B. G., Dubin M. W. and Levick W. R. (1971) Sustained and transient neurones in the cat’s retina and lateral geniculate nucleus. J. Physiol. 217. 473-496. Cleland B. G.. Levick W. R. and Sanderson K. J. (1973) Properties of sustained and transient cells in the cat retina. J. Physio~. 228. 649-680. Dow B. M. (1974) Functional ctasses of cells and their laminar distribution in monkey visual cortex. J. Neurophysiol. 37. 927-946.
Kolers P. A. (1972) Aspects of Marion Percrpcion. Pergamon Press. New York. Kolers P. A. and von Grlinau M. (1976) Shape and color in apparent motion. Vision Res. 16. 329-335. Kolers P. A. and von Griinau M. (1977) Fixation and attention in apparent motion. Q. J1 exp. Psychol. 29. 389-395.
Korte A. (1915) Kinematoskopische
Untersuchungen.
Z.
Psychol. 72. 194-296.
Kulikowski J. J. and Tolhurst D. J. (1973) Psychophysical evidence for sustained and transient detectors in human vision. J. P~ys~o~.232, 149-162. Grlansky J. (1940) The effect of similarity and difference in form on apparent visual movement. Archs Psychol.. N.Y. 246. l-88.
Singer W. and Bedworth N. (1973) Inhibitory interaction between X and Y units in the cat lateral geniculate nucleus. Bruin Res. 49. 291-307. Tolhurst D. J. (1973) Separate channels for the analysis of the shape and the movement of a moving visual stimulus. J. Physiol. 231. 385-402. Wlssle H.. Levick W. R. and Cleland 8. G. (1975) The distribution of the alpha type of ganglion cells in the cat’s retina. J. camp. Neural. 159. 419-437. Wertheimer M. (1912) Experimentelfe Studien iiber das Sehen von Bewegung. 2. Psychol. 61. 161-265.
Max-Planck-Institut
W. VON GRLWAU
fiir Psychiatric. Kraepelinstr. 2. 8 Miinchen 40. Germany
(Rrcrired 16 Febrwry
1977: in revisedform 25 &fay 1977)
Abstract-The effect of changing spatial frequency content of the inducing flashes on the likelihood of seeing apparent motion was investigated The results suggest that the sustained channel inhibits the transient channel. This inhibitory effect is largest in the fovea and decreases sharply towards the periphery. Conditions that weaken the sustained channel lead to a decrease of the inhibitory effect and a corresponding increase in the response of the transient channel.
INTRODL’CY’IO’ION
Much psychophysi~l and neurophy~olo~~l evidence has been accumulated over the past ten years. demonstrating the existence of two parallel processing
systems in the visual system, the transient and sustained channels (e.g. Enroth-Cugell and Robson, 1966; Cleland. Dubin and Levi&, 1971; CIeland, Levick and Sanderson, 1973; Ike& and Wright, 1974; 1975; Kulikowski and Tolhurst 1973; Tolhurst. 1973). The transient channel is characterized by high temporal resolution (responding optimally to fast movement, flicker, abrupt stimulus on- and offsets etc.). but low spatial resolution (mainly concerned with low spatial frequencies). The sustained channel, on the other hand, shows low temporal resolution (responding optimally to slow movement, stationary targets, prolonged presentations, etc.), but high spatial resoiution (preferring high spatial frequencies) The transient channel therefore is thought to process information about stimulus location and change of location or motion, while the sustained channel processes information about the spatial detail or form of the stimulus. The two systems are not inde~dent, however. Singer and Bedworth (1973) have demonstrated that the sustained channel is inhibited by the transient channel, and psychophysical experiments on apparent motion implicate reciprocity in these interactions. since. on the one hand, motion isolation inhibits form information (Breitmeyer, Love and Wepman. 1974; Breitmeyer, Battaglia and Weber, 1976) and, on the other hand, form information has been shown to affect both the color of the apparently moving object (Kolers and von Griinau, 1976) as well as the likdihood of perceived motion (Orlansky, 1940; Frisby, 1972; Kolers, 1972). The way in which the sustained channel influences the transient channel has not yet been clarified, however, neither neurophysiologically nor psychophysically. In the present report the interaction between the two channels in apparent motion is studied by mani’ This work was partially supported by funds from the Deutsche Forschungsgemeinschaft SFB 50, Kybemetik. 197
pulating the relative stimulus effectiveness in exciting the two channels. This is done by changing the spatial frequency content and the retinal position of the inducing flashes. METHODS Appararus
A schematic representation of the 3-channel Maxwellian View system which was used for the presentation of the stimuli is shown in Fin. la. The linht sources consisted of IO0 W Halogen lamp<. The shutteis were EEG magnets whose timing was controlled by electronic circuitry. Intensity of the fight beams was regulated by insertion of neutral density filters. The observer viewed the display with his right eye through an exit pupil of 2 mm dia. A small additional 4-D lens could be inserted in front of it. The effect of this lens was to defocus the stimuli. i.e. to reduce their high spatial frequency content. The observer’s head was stabitized by in~vid~lly fitted bite bars, The stimuli were dark 0.5’ squares on a light (1.51~~) background field of 7” dia. Thev aooeared one at a time 1.4” above the horizontal midline df’the display (Fig. I& Their separation(S) was I”, 2” and 4’. A dark fixation point was presented halfway between the two stimuli, either on the midline (0’) or 2” below. The stimuli were flashed for 100 msec each with a pause between them (ISI) of either 0, 25. 50, 75 or IOOmsec. The interval between the offset of the second and the following onset of the first stimulus (ICI) was set at l..Ssec, so that apparent motion occurred in the left-to-right direction only. Procedure The likelihood of seeing motion was measured for 3 different separations of the inducing flashes with central fixation. The inducing flashes thus came to lie in the foveai region (S = I”), the parafovea (S = 2”) and the near periphery (S = 4’). while the motion path always had to cross the fovea. Likelihood of motion was also measured at a separation of 2’ when the fixation point was 2” below the motion path, so that the inducing flashes as well as the motion path came lo lie extrafoveally. All these measurements were done both with and without the defocusing lens. Within these 8 conditions 40 presentations were given at each of 5 values of ISI. Conditions and values of IS1 appeared in a predetermined randomized order. Before every session the observer was dark-adapted for about 20min and then light-adapted for a few minutes
198
Fig. 1. (a) Schematic representation of the 3-channel Maxwellian View system. (b) Stimulus display. Only one flash was present at any one time. S = separation, FP = ftxation point. to the average luminance level of the display. After neglecting the first 3 responses an average of 5 responses were recorded at one ISI before switching to the next. After every presentation (during the ICI) the observer responded with a “yes” or “no” to the question: Do you see smooth continuous motion of one object across the whole distance?
The two observers had good color and spatial vision as tested with the H-R-R Pseudo-isochromatic plates and the Bausch & Lomb Orthorater. They were both well-prac-
ticed in psychophysical experiments. and LW was completely unaware of the purpose of this study. RESULTS
Analyses of variance were performed on the results separately for the two observers. They yielded highly significant effects at the 0.01 level for all variables and interactions, except that for observer LW the Iqns effect is significant at the 0.05 level and the lens-
C
./’
[ t
..Y
.
I 0
is
50
7s
1
i 100
ISI
0
2s
SO
7s
Cmsl
100
0 Fmttmn
2
01 FP [“I
Fig. 2. Likelihood of seeing apparent motion as a function of ISI for two observers. when fixation is in the center (A, D) or in the periphery (B, E). C and F: Overall effect of fixation point. o--_-o without lens, O--+
with lens.
Interaction between sustained and transient channels
separation interaction is not significant. Because of the significance of the triple interaction, further tests on the simple 2-way interactions were done. Relevant simple main effects were also tested. Plotting likelihood of motion as a function of ISI leads in all cases to the characteristic inverted-U relationship. Figure 2 shows some curves of this kind with absence or presence of the defocusing lens as parameter. For both observers the left graph (A and D) is for a separation of 2” at the central fixation point, while the middle graph (B and E) is for the same separation, but for a fixation point 2” below the motion path. The interactions in A and D are both highly significant (P < 0.01). while the interactions in B and E are not significant. This means that in A and D the curves for the condition with the lens are shifted toward shorter ISI, while in B and E they are essentially parallel. Aside from the shift toward shorter ISI, introduction of the defocusing lens also leads to a significant (P < 0.05 for LW. P < 0.01 for MG) overall increase in the likelihood of seeing motion, but only when fixation is central and not when it is peripheral (C and F). Defocusing also has differential effects depending on the separation between the two inducing flashes: The effect of the lens (shift toward shorter IS1 and increase in likelihood of motion) is large for the two small separations, when the inducing flashes lie in fovea1 and parafoveal regions. but almost disappears at the large separation. This is graphed in Fig 3 for both observers. Moving the fixation point from the center to 2” below the midline, has an effect similar to defocusing. Without the lens, moving to the periphery shifts the likelihood-of-motion vs IS1 relationship toward shorter ISI and increases the likelihood of seeing motion. With the lens there is only a small increase in the likelihood of motion and no shift (Fig. 4).
10
LW
199
The separation between the two inducing flashes influences the likelihood of seeing motion only insofar as the smallest separation, i.e. when both flashes lie in the fovea, leads to significantly more reports of apparent motion for both observers than the other two separations (see Fig 3). DISCUSSION The effect of the extra lens was to defocus the inducing flashes. In terms of spatial frequency content this means that high frequencies were largely removed from the spectrum, while low frequencies were hardly affected. The effective stimulus situation for the transient channel therefore was not changed and one would not expect any changes in its response. i.e. the likelihood of apparent motion or its temporal parameters should remain the same. The effective stimulus situation for the sustained channel, however, was changed drastically. One would expect that its response is reduced considerably, including inhibitory effects that it might have onto the transient channel. This would have the effect of weakening the processing of form information, while it would simultaneously strengthen the processing of motion information. This again would result in impaired form perception and improved perception of motion. That defocusing has differential effects on the transient and sustained channels was shown by Ikeda and Wright (1972) While the responsiveness of sustained cells was reduced considerably even by small amounts of blurring, the responsiveness of transient cells was relatively unaffected even by well-defocused stimulus images. Thus a 4-D defocusing lens was found to affect both channels in a drastically different way. Form perception was not tested here, but the present experiment demonstrated quite clearly that likelihood of seeing motion is increased when the in-
A
0
Fig. 3. Likelihood of seeing apparent motion as a function of ISI for two observers, when separation between the two inducing flashes is 1” (A, D), 2” (B, E) or 4’ (C. F). W without lens, M with lens.
MICHAEL W. LO? GR~L.AL
ISI [ml
Fig. 4. Likelihood of seeing apparent motion as a function of ISI for two observers without (A, C) and with (B. D) the defocusing lens. O--O fixation in the periphery, +---o fixation in the center.
ducing flashes are defocused. Furthermore, motion occurs at shorter intervals between the two inducing flashes, i.e. processing is done about 25 msec faster. This corresponds well with the temporal differences between the transient and sustained channels as reported in the physiological literature (Dow, 1974; Fukuda, 1973; Ikeda and Wright, 1975) and might reflect a greater prominence of the transient channel with its shorter latencies. For a gross perceptual event like apparent motion one must assume that a certain amount of averaging between short- and long-latency responses takes place. Thus motion occurs over a large range of ISI, and a shift of this distribution towards shorter ISI means that the relative importance of short-latency responses is increased Further analysis, however, shows that defocusing has its effects only when fixation is in the center. With peripheral fixation both effects disappear. This finds its explanation in the fact that the ratio of transient to sustained units is somewhat larger in the periphery than it is in the central fovea (Wiissle, Levick and Cleland, 1975; Fukuda and Stone, 1974). The inhibitory effect of the sustained channel onto the transient channel therefore will be weaker in the periphery. This, of course, leads to an increase in the likelihood of motion in this area (see Kolers and von Griinau, 1977, and the present Fig 4). Defocusing of the inducing flashes here has no additional effect, since the sustained channel is already of much less importance. The reciprocity between changing the frequency content or changing the retinal position of the inducing flashes also becomes clear from Fig 4. When the flashes are focused (A and C), changing fixation point from center to periphery brings an increase in motion as well as a shift to shorter ISI. When the &&es are defocused, however, changing fixation point has only a slight effect. It follows that changing fixation
point, i.e. moving into a peripheral, less form-sensitive area, can have an effect only when the sustained channel is not already weakened by e.g. defocused stimuli. Leaving the fixation point in the center, but moving the inducing flashes into the periphery by changing their separation leads to effects that are not quite as straightforward. On the one hand, the effects of the defocusing lens almost disappear at the large separation, in line with the above argument On the other hand, apparent motion is seen most often with the smallest separation, when the inducing flashes he in the fovea, but still less than with peripheral fixation. Two more factors have to be considered here. First. separation between the inducing flashes as such has been shown to all&t the system’s ability to create apparent motion (Wertheimer, 1912; Korte. 1915) Thus motion is generally seen more readily when the flashes are closer together. Secondly, it was observed in Kolers and von Griinau (1977) that the likelihood of motion is decreased when motion has to go through the fovea, even if the inducing stimuli he well in the periphery. The retinal position not only of the inducing gashes but also of the motion path seems to determine whether motion is seen or not. In the present optical system the extra lens leads to a slight reduction of image size. This effect is ap proximately compensated by the enlargement due to defocusing. Overall this results in stimulus images of roughly the same area with, however, slightly reduced contrast. According to Korte’s Law (Korte, 1915) this should give a slight increase of optimal ISI. working against the eflbct observed here. that optimal ISI shifted towards lower values with the extra lens In summary, the results of the present experiment suggest that the sustained channel inhibits the transient channel. This inhibitory ef?&t is largest in the fovea and decreases sharply towards the periphery. Conditions that weaken the sustained channel (like
201
interaction between sustained and transient channels defocusing of the stimuli) lead to a decrease of the inhibitory effect and a corresponding increase in the response of the transient channel. It appears therefore that the interactions between
the two channels are symmetrical, each channel inhibiting the other. Both interactions also seem to occur concurrently since during apparent motion the second inducing flash has been shown to have a metacontrast effect on the first fiash (Breitmeyer, Battaglia and Weber, 1976). This kind of masking effect has been linked by Breitmeyer and Ganz (1976) to an inhibition of the first flash’s sustained response by. the second flash’s transient response. A similar inhrbrtory effect of the first flash’s sustained response onto the second gash’s transient response is suggested by the present results. Direct physiological evidence is not available as yet, but strong indications for the existence of such an interaction have been observed (Singer, personat
co~uni~tion~
Enroth-Cugell C. and Robson J. G. (1966) The contrast sensitivity of retinal ganglion cells of the cat. J. Physiol. 187. 5 17-552. Frisby J. P. (1972) The effect of stimulus orientation on the phi phenomenon. Vision Res 12, 1145-l 166. Fukuda Y. (1973) Differentiation of principal cells of the rat lateral geniculate body into two groups: fast and slow cells. Exul Brain Res. 17. 242-260. Fukuda Y. and’ Stone J. (1974) Retinal distribution and central projections of Y-. X- and W-cells of the cat’s retina J. Ntwrouhvsioi. 37. 749-772. Ikeda H. and Wright M. J. (1972) Differential effects of refractive errors and receptive field organization of central and peripheral ganglion cells. Vision Res. 12, 1465-1476. Ikeda H. and Wright M. J. (1974) Evidence for “sustained” and “transient” neurones in the cat’s visual cortex. Vision Res. 14. 133-136. Ikeda H. and Wright M. J. (1975) Spatial and temporal properties of “sustained” and “transient” neurones in area 17 of the cat’s visual cortex. Expl Brain Res. 22. 363-383.
Acknowfedgemmrs-I wish to thank Dr. W. Singer for the opportunity to conduct this research and for his valuable advice, Dip.-Ing. G. Neumann for technical assistance with the electronics and LW for being a patient observer. REFERENCES
Breitmeyer B. G. and Ganz L. (1976) Implications of sustained and transient channels for theories of visual pattern masking. saccadic suppression, and information processing. Psychof. Rea 83, l-36. Breitmeyer B. G., Love R. and Wepman B. (1974) Contour suppression during stroboscopic motion and metacontrast. Yisiott Res. 14, 1451-1456. Breitmeyer B. G., Battaglia F. and Weber C. (1976) U-shaped backward contour masking during stroboscopic motion. J. exp. P&ol. Human Perception and Performawe 2. 167-I 73. Cleland B. G., Dubin M. W. and Levick W. R. (1971) Sustained and transient neurones in the cat’s retina and lateral geniculate nucleus. J. Physiol. 217. 473-496. Cleland B. G.. Levick W. R. and Sanderson K. J. (1973) Properties of sustained and transient cells in the cat retina. J. Physio~. 228. 649-680. Dow B. M. (1974) Functional ctasses of cells and their laminar distribution in monkey visual cortex. J. Neurophysiol. 37. 927-946.
Kolers P. A. (1972) Aspects of Marion Percrpcion. Pergamon Press. New York. Kolers P. A. and von Grlinau M. (1976) Shape and color in apparent motion. Vision Res. 16. 329-335. Kolers P. A. and von Griinau M. (1977) Fixation and attention in apparent motion. Q. J1 exp. Psychol. 29. 389-395.
Korte A. (1915) Kinematoskopische
Untersuchungen.
Z.
Psychol. 72. 194-296.
Kulikowski J. J. and Tolhurst D. J. (1973) Psychophysical evidence for sustained and transient detectors in human vision. J. P~ys~o~.232, 149-162. Grlansky J. (1940) The effect of similarity and difference in form on apparent visual movement. Archs Psychol.. N.Y. 246. l-88.
Singer W. and Bedworth N. (1973) Inhibitory interaction between X and Y units in the cat lateral geniculate nucleus. Bruin Res. 49. 291-307. Tolhurst D. J. (1973) Separate channels for the analysis of the shape and the movement of a moving visual stimulus. J. Physiol. 231. 385-402. Wlssle H.. Levick W. R. and Cleland 8. G. (1975) The distribution of the alpha type of ganglion cells in the cat’s retina. J. camp. Neural. 159. 419-437. Wertheimer M. (1912) Experimentelfe Studien iiber das Sehen von Bewegung. 2. Psychol. 61. 161-265.