The organization and development of the lateral suprasylvian visual areas of the cat visual cortex

Electrophysiological techniques were used to study the orderliness of representation of visual space in two areas in the medial (PMLS) and lateral (PLLS) banks of the middle suprasylvian sulcus in anaesthetized and paralysed cats. In some penetrations, more frequently in PMLS than in PLLS, clear topographic order was found in the representation of the visual field, but along many electrode tracks no retinotopic arrangement could be discerned.
Visual responses of units in PMLS and PLLS were assessed qualitatively for projected spot and bar stimuli and quantitatively for high-contrast sine-wave gratings. In both areas a very high proportion of direction selective cells was found, which were on average well 'tuned' for the direction of motion. Tests with stationary, flashed gratings of different orientation revealed that those neurones responsive to stationary stimuli were orientation selective and that their preferred orientations for stationary and drifting gratings were very similar. To study the functional architecture of the suprasylvian cortex single and multiple penetrations were made at different angles to the cortical surface. At least in most parts of PMLS there was evidence for a local sequential representation of stimulus orientation or axis of motion across the cortical surface.
The responses of neurones in PMLS and PLLS to gratings of optimum orientation and direction of motion, but varying in spatial and temporal frequency were analysed. The majority of neurones was band-tuned to spatial frequency (though a few, especially in PLLS, had low-pass spatial tuning) and the optimum spatial frequencies and 'acuities' were similar to those of cells in area 18. Neurones in PMLS and PLLS showed strong non-linearities in their responses to phase-alternating gratings and their spatial selectivity was usually determined by 'sub-units' smaller than the receptive field, much like complex cells in areas 17 and 18. Cells were broadly tuned to the temporal frequency of a drifting grating and there was a tendency for neurones in PLLS to prefer higher temporal frequencies (therefore faster velocities) than those in PMLS.
The postnatal development of receptive field properties of neurones in PMLS was studied quantitatively in kittens aged between 9 days and 8 weeks. In the youngest kittens the small population of visually responsive cells appeared to be distributed in patches that were restricted to the lower cortical layers and most of them already exhibited some degree of directional tuning. Direction and orientation selectivity matured rapidly and adult values for the proportions of direction and orientation selective cells were reached at the end of the third week of postnatal life. An improvement of spatial resolution of neurones in PMLS also occurred during the first three weeks, similar to that of cells in area 17, but over a shorter period. Temporal properties matured relatively slowly and had in some respects not reached adult values in the oldest kittens studied.
In summary, PMLS and PLLS seem to act as parallel visual areas specialized for the detection of motion that share more of their response properties with areas 17 and 18 than has been thought previously.

Advisor: Blakemore, C