SPATIAL AND TEMPORAL FREQUENCY SELECTIVITY IN VI, VII AND INFEROTEMPORAL CORTEX IN THE PRIMATE.

Abstract

We investigated the spatio-temporal frequency selectivity properties of 248 neurons in VI and VII of the macaque monkey, and the receptive field properties of 43 inferotemporal neurons of the owl monkey. The study of VI and VII neurons was carried out using sine wave gratings. Our results show that VI and VII neurons were selective to different but partially overlapping spatial frequency ranges (at retinal eccentricities of 2°-5°, VII neurons are tuned approximately two octaves lower than VI neurons). The spatial frequency bandwidths for the two visual areas are similar. The temporal frequency selectivity of VI neurons was mostly lowpass (70%), or wide bandpass (30%, mean bandwidth 3 octaves); 62% of VII neurons had bandpass temporal frequency tuning curves (mean bandwidth 2 octaves). The range of preferred temporal frequencies and the bandwidths of VII neurons suggest the existence of at least two non-overlapping temporal frequency filters at each spatial frequency. Results also indicate that spatial and temporal selectivities are independent fundamental properties. Our results may provide physiological support for psychophysical findings in man, suggesting several independent detection mechanisms centered at different temporal frequencies, especially at low spatial frequencies. In a different study, a possible substructure of the large inferotemporal receptive fields was investigated in the owl monkey. A novel kind of stimulus derived from the Gabor elementary functions was used as test signal. A first set of stimuli with a constant relative spatial frequency bandwidth was used to probe the existence of either one or more than one spatial frequency band within one receptive field. A second set of constant spatial width was used to test the neruon's spatial frequency selectivity. Results suggest that only one orientation and spatial frequency band project onto each neuron. The preferred spatial frequencies (0.2 - 0.6 c/deg) were in the very low spatial frequency range for this animal. The spatial frequency bandwidth comparable to that of striate cells, the inclusion of many cycles of the preferred spatial frequency within the receptive field, and the generally reduced response to constant aperture test signals, suggest non-linear processing during summation of information from preceding visual areas.