- Email: [email protected]
The visual evoked response dispersion curve: its relation to cerebral stochastic and deterministic mechanisms
FREE COMMUNICATIONSIN EEG made by placing a small strip of silver between the focus and the slab (4-5 mm away from the epileptogenic focus), decrease by placing a grounded silver strip between the two. It was found that an increase in conductivity raised the amplitude of potentials in the slab from 68/.tV to 143 pV. Decrease in conductivity reduced the amplitude to 45 pV. Activity from 430 neurons was recorded in the slab. With an increase in conductivity the number of excited neurons increased by 10% and no changes were demonstrated on the inhibited cells. With decrease in conductivity the number of excited neurons decreased by 14% and the inhibited cells by 4.5%. The findings indicate that the ephaptic factor is more effective when the resistance to current spread decreases and demonstrate the effects of passive conductivity of biocurrents. They also confirm our suggestion of the part played by electrical fields in the synchronization or neuronal activity. 126. The visual evoked response dispersion curve:
its relation to cerebral stochastic and deterministic mechanisms.-V. Constantineseu and E. Crighel (Bucharest, Rumania). Some of the mechanisms involved in the process of information transmission are stochastic in character. Such mechanisms, generally, do not start from an always identical initial state, but from the random state existing at the m o m e n t of appearance of the signal They subsequently develop in a stochastic way. On these grounds the authors consider that averaging techniques and those derived from it have the disadvantage of eliminating precisely the stochastic components considered as noise and retaining the deterministic ones which carry much less information. Therefore, as a further investigative method, the following was devised and tested. By statistical processing of evoked responses (ER) groups in man and cat, under different physiological conditions, the variation of dispersion with time was computed and plotted as a curve D = f(t) with three distinct periods. In man the first period following the signal was characterized by a very large dispersion with a latency of 35 to 40 msec and lasting for 130 msec, the second lasted 100 to 170 msec with a small dispersion and the third exhibited again a high dispersion. In cat the same dispersion curves were found but with shorter latencies. Taking into account the constant character of the flash and the equiprobability of the stochastic external disturbances, the authors consider that the dispersion curves of the ER indicate that the
699
biologic system involves some mechanisms answering in a deterministic way and some answering in a stochastic way to external signals, and that in each of the three periods one or the other type of mechanism is prevalent. These results are in agreement with physiological data and could form the basis for a mathematical model. It is also possible to consider that the dispersion D at a given moment is the resultant of three components: the variation of (i) latencies, (ii) amplitudes and (iii) patterns of the analyzed group of ERs. The global curve D = f(t) seems, however, to be more useful for interpretation and closer to the biologic phenomenon than the splitting in these three components.
127. The amplitude of slow evoked cortical responses in relation to temporal features of stimulation.-H. H. Rothman and H. Davis (St. Louis, Mo., U.S.A.). A series of experiments were run to determine how slow evoked cortical responses were affected by temporal features of auditory and shock stimulation, in human subjects. The response was recorded extracranially between vertex and right mastoid. Amplitude was measured from the 100 msec negative peak (N 1) to the 200 msec positive peak (P2)' One experiment explored how irregular interstimulus intervals affected responses, compared to strictly regular intervals. Irregularity generaUy enhanced response amplitude though its specific effect depended on the particular subject and modality, and certain subjects did not consistently produce the effect at each session. A slightly modified experiment with shocks alone showed a more consistent effect. Also, in the modified experiment, response recovery was recorded as a function of the interval between the successive shocks. Both regular and irregular schedules of stimulation were used. The shapes of the recovery curves were quite similar for both schedules, but the absolute amplitudes were slightly larger for irregular presentation, in accord with the findings above. The modified experiment also furnished data regarding the effects of the immediate preceding interval on response amplitude, compared to the next-earlier preceding interval. The next-earlier preceding intervals were found to have no discernible effect for the subject group as a whole (n=4). In addition, if the immediate preceding interval was short (about 0.5 sec) then an increase in background stimulation rate caused a profound decrease in response amplitude. The findings suggest that the slow evoked response is influenced by the most recent preceding event, and by a summation of events earlier
its relation to cerebral stochastic and deterministic mechanisms.-V. Constantineseu and E. Crighel (Bucharest, Rumania). Some of the mechanisms involved in the process of information transmission are stochastic in character. Such mechanisms, generally, do not start from an always identical initial state, but from the random state existing at the m o m e n t of appearance of the signal They subsequently develop in a stochastic way. On these grounds the authors consider that averaging techniques and those derived from it have the disadvantage of eliminating precisely the stochastic components considered as noise and retaining the deterministic ones which carry much less information. Therefore, as a further investigative method, the following was devised and tested. By statistical processing of evoked responses (ER) groups in man and cat, under different physiological conditions, the variation of dispersion with time was computed and plotted as a curve D = f(t) with three distinct periods. In man the first period following the signal was characterized by a very large dispersion with a latency of 35 to 40 msec and lasting for 130 msec, the second lasted 100 to 170 msec with a small dispersion and the third exhibited again a high dispersion. In cat the same dispersion curves were found but with shorter latencies. Taking into account the constant character of the flash and the equiprobability of the stochastic external disturbances, the authors consider that the dispersion curves of the ER indicate that the
699
biologic system involves some mechanisms answering in a deterministic way and some answering in a stochastic way to external signals, and that in each of the three periods one or the other type of mechanism is prevalent. These results are in agreement with physiological data and could form the basis for a mathematical model. It is also possible to consider that the dispersion D at a given moment is the resultant of three components: the variation of (i) latencies, (ii) amplitudes and (iii) patterns of the analyzed group of ERs. The global curve D = f(t) seems, however, to be more useful for interpretation and closer to the biologic phenomenon than the splitting in these three components.
127. The amplitude of slow evoked cortical responses in relation to temporal features of stimulation.-H. H. Rothman and H. Davis (St. Louis, Mo., U.S.A.). A series of experiments were run to determine how slow evoked cortical responses were affected by temporal features of auditory and shock stimulation, in human subjects. The response was recorded extracranially between vertex and right mastoid. Amplitude was measured from the 100 msec negative peak (N 1) to the 200 msec positive peak (P2)' One experiment explored how irregular interstimulus intervals affected responses, compared to strictly regular intervals. Irregularity generaUy enhanced response amplitude though its specific effect depended on the particular subject and modality, and certain subjects did not consistently produce the effect at each session. A slightly modified experiment with shocks alone showed a more consistent effect. Also, in the modified experiment, response recovery was recorded as a function of the interval between the successive shocks. Both regular and irregular schedules of stimulation were used. The shapes of the recovery curves were quite similar for both schedules, but the absolute amplitudes were slightly larger for irregular presentation, in accord with the findings above. The modified experiment also furnished data regarding the effects of the immediate preceding interval on response amplitude, compared to the next-earlier preceding interval. The next-earlier preceding intervals were found to have no discernible effect for the subject group as a whole (n=4). In addition, if the immediate preceding interval was short (about 0.5 sec) then an increase in background stimulation rate caused a profound decrease in response amplitude. The findings suggest that the slow evoked response is influenced by the most recent preceding event, and by a summation of events earlier