Guy Orban

Influence of retinal lesions on grating acuity of the cat

Grating acuity was investigated behaviorally in the cat. Retinal lesions of increasing diameter centered on the area centralis were made by photocoagulations in one eye, while the intact eye was used as control. Lesion size evaluated from fundus photographs was precisely correlated with the anatomical lesion size measured in retinal whole mounts. Grating acuity improved with increasing grating area in cats with intact retinae and after small (less than 3 degrees diameter) retinal lesions but not after large lesions (greater than 4 degrees diameter). Overall, grating acuity clearly decreased when lesions became larger than 4 degrees in diameter. The acuity-eccentricity relationship closely fits the cutoff frequency of brisk sustained cells at the corresponding eccentricities.

Contrast modulated steady-state visual evoked potentials (CMSS VEPs): recording evoked potentials and related single cell responses in area 17 of the cat

(1) The functional characteristics of the neuronal substrate, responding to the CMSS VEP stimulus, were studied by recording CMSS VEPs and related single unit activity in area 17 of the anaesthetised and paralysed cat. CMSS VEPs use an 8 Hz phase reversing (i.e., 16 reversals/sec) grating stimulus with rapid contrast sweeping and allow the contrast thresholds and lag values to be measured as a function of the spatial frequency. (2) The CMSS VEPs of the anaesthetised cat have a wave form similar to those of humans but are shifted to lower spatial frequencies, higher contrast thresholds and longer lag values. (3) The cellular response to a sinusoidal grating, phase reversing at 8 Hz, was studied in order to identify the neuronal substrate generating the CMSS VEPs. Sixty percent of the area 17 cells respond to this stimulus. Cells responding at 8 Hz reversal comprise a distinct subpopulation of visual cortical cells selective for higher velocities and lower spatial frequencies. (4) Although the CMSS VEPs contain almost exclusively energy at 16 Hz, the temporal response pattern of striate cells is quite disparate, including first and second harmonic response patterns as well as an intermediate type. (5) There is a near-perfect correlation between the contrast thresholds of single cells, obtained with the contrast swept stimulus and those obtained with a static contrast test, validating the technique of rapid linear contrast sweeping. (6) The influence of the temporal parameters of the contrast sweeping on the calculated contrast threshold was investigated at the neuronal level. These parameters only marginally influence the responses. (7) CMSS VEP contrast thresholds and neuronal thresholds were compared. The sensitivity of VEPs corresponds to that of the most sensitive neuronal generators. CMSS VEP lag values are longer than the values for individual neurones.