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

1. Single cortical neurons are known to respond to visual stimuli cont aining Fourier components only in a narrow range of spatial frequencie s. This investigation demonstrates that some neurons in cat area 17 an d 18 can also respond to certain stimuli that have no Fourier componen ts inside the cell's luminance spatial frequency passband. 2. To study such ''non-Fourier'' responses, we used envelope stimuli that consist ed of a high-spatial-frequency sinusoidal luminance grating (carrier) whose contrast was modulated by a low-spatial frequency sine wave (env elope). 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 Fo urier components in the stimuli were outside.3. We conducted extensive control experiments to eliminate the possibility of producing artifac tual responses to the envelope stimuli due to any small residual nonli nearity of the z-linearized CRT screen. The control experiments includ ed I)testing of screen linearity to ensure that the effect from the re sidual screen nonlinearity was no larger than the sensitivity level of visual responses and 2) comparing the responses to envelope stimuli w ith the responses to the equivalent contrast of the artifact produced by the screen nonlinearity. All these control experiments indicated th at any effect of screen nonlinearity did not contribute significantly to the neural envelope responses. 4. We performed a statistical analys is to obtain an index of relative strength of envelope responses for e ach cell and to objectively classify cells as ''envelope-responsive'' or ''non-envelope-responsive.'' A clear segregation between envelope-r esponsive and non-envelope-responsive cells was observed in the distri bution of relative envelope response strength. 5. The distribution of envelope-responsive cells exhibited a bias between the two cortical ar eas and between simple versus complex cell types in area 17. About hal f of the simple cells and most of the complex cells in area 18 were en velope responsive, whereas only 1 of 12 simple and a minority of compl ex cells in area 17 were. 6. The strength of envelope responses was ge nerally 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, orientat ion selectivity, and temporal modulation. 7. The existence of such env elope-responsive cells implicates areas 17 and 18 as a neural basis fo r the early processing of non-Fourier aspects of visual information th at have been extensively demonstrated by human psychophysics.