ENVELOPE-RESPONSIVE NEURONS IN AREA-17 AND AREA-18 OF CAT

1. Single cortical neurons are known to respond to visual stimuli containing Fourier components only in a narrow range of spatial frequencies. This investigation demonstrates that some neurons in cat area 17 and 18 can also respond to certain stimuli that have no Fourier components inside the cell's luminance spatial frequency passband. 2. To study such ''non-Fourier'' responses, we used envelope stimuli that consisted of a high-spatial-frequency sinusoidal luminance grating (carrier) whose contrast was modulated by a low-spatial frequency sine wave (envelope). There was no Fourier component at the apparent periodicity of the envelope spatial frequency. However, some cells responded to such a ''phantom'' component of the envelope modulation when it fell inside the cell's luminance spatial frequency passband while all the real Fourier components in the stimuli were outside. 3. We conducted extensive control experiments to eliminate the possibility of producing artifactual responses to the envelope stimuli due to any small residual nonlinearity of the z-linearized CRT screen. The control experiments included I)testing of screen linearity to ensure that the effect from the residual screen nonlinearity was no larger than the sensitivity level of visual responses and 2) comparing the responses to envelope stimuli with the responses to the equivalent contrast of the artifact produced by the screen nonlinearity. All these control experiments indicated that any effect of screen nonlinearity did not contribute significantly to the neural envelope responses. 4. We performed a statistical analysis to obtain an index of relative strength of envelope responses for each cell and to objectively classify cells as ''envelope-responsive'' or ''non-envelope-responsive.'' A clear segregation between envelope-responsive and non-envelope-responsive cells was observed in the distribution of relative envelope response strength. 5. The distribution of envelope-responsive cells exhibited a bias between the two cortical areas and between simple versus complex cell types in area 17. About half of the simple cells and most of the complex cells in area 18 were envelope responsive, whereas only 1 of 12 simple and a minority of complex cells in area 17 were. 6. The strength of envelope responses was generally smaller than that of responses to luminance grating stimuli at the same contrast. However, both the envelope and luminance responses were consistent for a given neuron in direction selectivity, orientation selectivity, and temporal modulation. 7. The existence of such envelope-responsive cells implicates areas 17 and 18 as a neural basis for the early processing of non-Fourier aspects of visual information that have been extensively demonstrated by human psychophysics.