- Email: [email protected]
An MEG study of the dynamics of human face processing
Society proceedings/
Electroencephalography and clinical Neurophysiology 98 (1996) 2P-4P
netic response to isoluminant chromatic stimuli with the response evoked by luminance. modulated stimuli. Evoked magnetic responses were recorded using a 19-channel magnetometer positioned over the occipital pole. The stimuli were 1 c/deg horizontally oriented gratings subtending 4 X 6” and displaced 0.5” from the principle meridians in each quadrant of the visual field. Red/green gratings were combined either in phase to produce yellow/black gratings or 180”out of phase to produce red/green gratings. Achromatic gratings of 80% contrast were also usled. Localisation of the response was achieved by equivalent current dipole (ECD) analysis and co-registration with each subject’s MRI. The response to isoluminant chromatic stimulation produced a high field strength component at ‘92-110 msec which was retinotopically organised in agreement with the cruciform model of striate cortex. ECD localisation indicated a Vl source. A later component at 180-260 msec was suggestive of V4 activation. The response to luminance stimuli was of low field strength and comprised 2-4 components which were not retinotopically organ&d. ECD Iccalisations were consistent with activation of Vl and V2. Chromatic and luminance stimuli evoke different spatio-temporal patterns of activation in human visual cortex consistent with their segregation at an early stage of cortical processing. (MEG) and stereopsis - A.I. Weir ‘, U. Shahani ‘, G. Lang b, D.C. Mansfield I, D. Hutson I, P.M. Maas ’ and G.B. Donaldson * (’ Southern General Hospital, Southampton, and b Strathclyde University, Glasgow)
5. Magnetoencephalography
Free running magnetoencephalograms were recorded from over the occipital and parietal cortices in humans in order to study the frequency of cortical activity during stereoscopic vision. Points O,,, and P,., of the international lo-20 electrode system were used to locate a single channel second order SQUID neuromagnetometer. Sequential recordings were made of subjects with eyes closed, eyes open and when perceiving the cyclopean wallpaper pattern under isoluminant conditions. Data were recorded on digital tape and analysed off-line in the frequency domain. In the eyes closed condition, the dominant frequencies were between 8 and 12 Hz attenuating on eye opening. With perception of the 3-dimensional image, this alpha rhythm was also attenuated. However, there was a marked increase at several frequencies between 35 and 75 Hz. There was substantial inter-individual variation of peak frequency, but for a constant location intra-individual variation was small. Our results are consistent with reports in the literature which suggest that frequencies above 40 Hz are involved in focussed attention, pattern recognition and higher order visual activity. 6. Magneto-encephalographic analysis of cortical area V5 in humans. - SJ. Anderson, I.E. Holliday, K.D. Singh and G.F.A. Harding (Department of Vision Sciences, Aston University, Birmingham)
Evidence for the existence of an area within the human visual cortex specialised for the analysis of motion has come from behavioural studies on brain-damaged patients, invasive studies during pre-surgical evaluations, and most recently from measurements of cortical activity using positron emission tomography (PET). This area is located near the occipito-temporal border just below the start of the ascending limb of the superior temporal sulcus, and is thought to be the human homologue of the primate cortical area V5 (MT). Our aim was to determine the spatio-temporal and contrast response properties of this area in humans using magneto-encephalography (MEG) which, in contrast to PET, provides a direct measure of electrical activity in the brain with a temporal resolution equal to the dynamics of cortical processing. To activate motion centres in the brain we used drifting achromatic grating stimuli, Gaussian-damped in 2-dimensional space. The magnetometer was positioned over the left occipito-temporal lobe of each subject. A source was located in the region of cortex previously identified
P3
as human VS (Z&i et al., 1991). We determined the response characteristics of this area by examining the variation in the magnitude of fitted equivalent current dipoles as a function of stimulus contrast, spatial frequency and drift temporal frequency. The area was selective for low spatial frequencies ( < 2.0 c/&g), responded to a wide range of temporal frequencies (< 40 Hz) and showed response saturation for stimulus contrasts > 10%. In addition, the area was responsive to motion-contrast patterns but was not responsive to purely chromatic patterns. Our results are consistent with the hypothesis that the extra-striate area we have identified represents a stage of motion processing within the magnocellular pathway of the human visual system. 7. An MEG study of the dynamics of human face processing. - SJ. Swithenby “, AJ. Bailey b, S.E. Brautogam ‘, O.E. Josephs ‘, V. Jousm%ki ’ and C. Tesche ’ (’ Open University, Milton Keynes, b MRC Child Psychiatry Unit, Maudsley Hospital, London, and ’ Helsinki University of Technology, Helsinki, Finhmd) This is a study of neural processing associated with briefly presented images of human faces. A 122channel whole-head MEG system was used in studies of 6 normal subjects who carried out 4 tasks in which they identified: (i) randomly presented human faces in a sequence of different images: (ii) identical pairs of faces amongst a series of face pairs, (iii) faces displaying a particular emotion in a series of faces; and (iv) the same individual amongst pairs of photographs where the individuals in tbe pair show different emotions. In each case, the subject carried out control tasks and gave a motor response via a non-magnetic keypad. We observed face-specific signals (bilaterally) at latencies from approximately 130 msec in all subjects. These were most distinct at early latencies and in right posterior and inferior signal channels. The early latency face-specific signals are largely independent of attention and the nature of the task. Their signal morphology is consistent with a complex source distribution in inferior occipito-temporal cortex. We will outline the extent to which our observations are consistent with previous studies using other techniques.
8. Multi-channel magnetometry at Aston University. - LE. Holiday and K.D. Sii Birmingham)
(Department of Vision Sciences, Aston University,
Recording magnetic fields from the human cortex requires the extreme sensitivity provided by SQUID magnetometry. Practical problems to be overcome include: (1) Environmental noise, reduced by magnetically shielded rooms and by the choice of gradiometer used in magnetometer design. At Aston a second-order design was chosen. Nevertheless, noise related to the electrical supply often remains and can be reduced by filtering, either analogue or digital. We also employ real-time adaptive filtering based on a vector magnetometer signal recorded synchronously with the acquired evoked magnetic field data. (2) Calibration. The use of MEG to localise cortical sources relies upon accurate system calibration. We have devised a grid based calibration procedure which yields suitable tolerances. (3) MRI co-registration is achieved with rigidly mounted dental bite bars with reference points visible in MRI and digit&d at the time of signal acquisition. (4) Signal analysis and source lccalisation. The MEG signals are viewed and contour mapped using software developed in-house. In addition, single and multiple dipole fitting programmes have been developed which allow any number of sources to be used to model the MEG signals. (5) MR reconstructions. As well as plotting source localisations on the raw MR slices, we have developed algorithms for rendering any view of the cortical surface. This allows the creation of a 3D computer generated model of the brain, with the functional localisations superimposed on its surface. This model can then be rotated and viewed from any angle.
Electroencephalography and clinical Neurophysiology 98 (1996) 2P-4P
netic response to isoluminant chromatic stimuli with the response evoked by luminance. modulated stimuli. Evoked magnetic responses were recorded using a 19-channel magnetometer positioned over the occipital pole. The stimuli were 1 c/deg horizontally oriented gratings subtending 4 X 6” and displaced 0.5” from the principle meridians in each quadrant of the visual field. Red/green gratings were combined either in phase to produce yellow/black gratings or 180”out of phase to produce red/green gratings. Achromatic gratings of 80% contrast were also usled. Localisation of the response was achieved by equivalent current dipole (ECD) analysis and co-registration with each subject’s MRI. The response to isoluminant chromatic stimulation produced a high field strength component at ‘92-110 msec which was retinotopically organised in agreement with the cruciform model of striate cortex. ECD localisation indicated a Vl source. A later component at 180-260 msec was suggestive of V4 activation. The response to luminance stimuli was of low field strength and comprised 2-4 components which were not retinotopically organ&d. ECD Iccalisations were consistent with activation of Vl and V2. Chromatic and luminance stimuli evoke different spatio-temporal patterns of activation in human visual cortex consistent with their segregation at an early stage of cortical processing. (MEG) and stereopsis - A.I. Weir ‘, U. Shahani ‘, G. Lang b, D.C. Mansfield I, D. Hutson I, P.M. Maas ’ and G.B. Donaldson * (’ Southern General Hospital, Southampton, and b Strathclyde University, Glasgow)
5. Magnetoencephalography
Free running magnetoencephalograms were recorded from over the occipital and parietal cortices in humans in order to study the frequency of cortical activity during stereoscopic vision. Points O,,, and P,., of the international lo-20 electrode system were used to locate a single channel second order SQUID neuromagnetometer. Sequential recordings were made of subjects with eyes closed, eyes open and when perceiving the cyclopean wallpaper pattern under isoluminant conditions. Data were recorded on digital tape and analysed off-line in the frequency domain. In the eyes closed condition, the dominant frequencies were between 8 and 12 Hz attenuating on eye opening. With perception of the 3-dimensional image, this alpha rhythm was also attenuated. However, there was a marked increase at several frequencies between 35 and 75 Hz. There was substantial inter-individual variation of peak frequency, but for a constant location intra-individual variation was small. Our results are consistent with reports in the literature which suggest that frequencies above 40 Hz are involved in focussed attention, pattern recognition and higher order visual activity. 6. Magneto-encephalographic analysis of cortical area V5 in humans. - SJ. Anderson, I.E. Holliday, K.D. Singh and G.F.A. Harding (Department of Vision Sciences, Aston University, Birmingham)
Evidence for the existence of an area within the human visual cortex specialised for the analysis of motion has come from behavioural studies on brain-damaged patients, invasive studies during pre-surgical evaluations, and most recently from measurements of cortical activity using positron emission tomography (PET). This area is located near the occipito-temporal border just below the start of the ascending limb of the superior temporal sulcus, and is thought to be the human homologue of the primate cortical area V5 (MT). Our aim was to determine the spatio-temporal and contrast response properties of this area in humans using magneto-encephalography (MEG) which, in contrast to PET, provides a direct measure of electrical activity in the brain with a temporal resolution equal to the dynamics of cortical processing. To activate motion centres in the brain we used drifting achromatic grating stimuli, Gaussian-damped in 2-dimensional space. The magnetometer was positioned over the left occipito-temporal lobe of each subject. A source was located in the region of cortex previously identified
P3
as human VS (Z&i et al., 1991). We determined the response characteristics of this area by examining the variation in the magnitude of fitted equivalent current dipoles as a function of stimulus contrast, spatial frequency and drift temporal frequency. The area was selective for low spatial frequencies ( < 2.0 c/&g), responded to a wide range of temporal frequencies (< 40 Hz) and showed response saturation for stimulus contrasts > 10%. In addition, the area was responsive to motion-contrast patterns but was not responsive to purely chromatic patterns. Our results are consistent with the hypothesis that the extra-striate area we have identified represents a stage of motion processing within the magnocellular pathway of the human visual system. 7. An MEG study of the dynamics of human face processing. - SJ. Swithenby “, AJ. Bailey b, S.E. Brautogam ‘, O.E. Josephs ‘, V. Jousm%ki ’ and C. Tesche ’ (’ Open University, Milton Keynes, b MRC Child Psychiatry Unit, Maudsley Hospital, London, and ’ Helsinki University of Technology, Helsinki, Finhmd) This is a study of neural processing associated with briefly presented images of human faces. A 122channel whole-head MEG system was used in studies of 6 normal subjects who carried out 4 tasks in which they identified: (i) randomly presented human faces in a sequence of different images: (ii) identical pairs of faces amongst a series of face pairs, (iii) faces displaying a particular emotion in a series of faces; and (iv) the same individual amongst pairs of photographs where the individuals in tbe pair show different emotions. In each case, the subject carried out control tasks and gave a motor response via a non-magnetic keypad. We observed face-specific signals (bilaterally) at latencies from approximately 130 msec in all subjects. These were most distinct at early latencies and in right posterior and inferior signal channels. The early latency face-specific signals are largely independent of attention and the nature of the task. Their signal morphology is consistent with a complex source distribution in inferior occipito-temporal cortex. We will outline the extent to which our observations are consistent with previous studies using other techniques.
8. Multi-channel magnetometry at Aston University. - LE. Holiday and K.D. Sii Birmingham)
(Department of Vision Sciences, Aston University,
Recording magnetic fields from the human cortex requires the extreme sensitivity provided by SQUID magnetometry. Practical problems to be overcome include: (1) Environmental noise, reduced by magnetically shielded rooms and by the choice of gradiometer used in magnetometer design. At Aston a second-order design was chosen. Nevertheless, noise related to the electrical supply often remains and can be reduced by filtering, either analogue or digital. We also employ real-time adaptive filtering based on a vector magnetometer signal recorded synchronously with the acquired evoked magnetic field data. (2) Calibration. The use of MEG to localise cortical sources relies upon accurate system calibration. We have devised a grid based calibration procedure which yields suitable tolerances. (3) MRI co-registration is achieved with rigidly mounted dental bite bars with reference points visible in MRI and digit&d at the time of signal acquisition. (4) Signal analysis and source lccalisation. The MEG signals are viewed and contour mapped using software developed in-house. In addition, single and multiple dipole fitting programmes have been developed which allow any number of sources to be used to model the MEG signals. (5) MR reconstructions. As well as plotting source localisations on the raw MR slices, we have developed algorithms for rendering any view of the cortical surface. This allows the creation of a 3D computer generated model of the brain, with the functional localisations superimposed on its surface. This model can then be rotated and viewed from any angle.