Phase-disparity coding in extrastriate area 19 of the cat

Abstract

Binocular interactions were investigated in area 19 of the anaesthetized cat using dichoptically presented phase-shifted static spatial frequency gratings that flickered at a fixed temporal rate. More than two-thirds of the binocular cells showed phase specificity to static phase disparities leading to either summation or facilitation interactions. This proportion of spatial disparity selectivity was higher than that shown for the same area (one-third of the units) when drifting light bars or drifting spatial frequencies were used to create disparities. The range of phase disparities encoded by binocular cells in area 19 is inversely related to the optimal spatial frequency of the dominant eye. Thus, cells in this area are tuned to coarse spatial disparities which, as supported by behavioural studies, could reflect its involvement in the analysis of stereoscopic pattern having gross disparities but devoid of motion cues. Because of the nature of its interconnections with numerous visual cortical areas, area 19 could serve as a way station where stereoscopic information could be first analysed and sent to other higher order areas for a complete representation of three-dimensional objects.

Figure 1. Spatial frequency properties of all cells recorded in area 19

Representative examples of spatial frequency tuning functions of the dominant eye for two complex cells (A and B). The stimuli used to derive the spatial frequency tuning functions were sinusoidal gratings (contrast: 30 %) flickered at a temporal frequency of 2 Hz (A) and 4 Hz (B). The distribution of the optimal spatial frequency (C) for the dominant eye of 62 cells shows that most units are sensitive to low spatial frequencies (mean = 0.18 cycles deg⁻¹). The spatial bandwidth distribution (D) of the dominant eye reveals that the cells are highly selective to static spatial frequency gratings (mean = 1.6 octaves).

Figure 2. Peristimulus time histograms and phase disparity tuning function for a typical phase-sensitive cell

The cell shows strong facilitation and an optimally modulated response at a phase disparity of 67.5 deg. Horizontal lines represent the monocular responses of the contralateral (contra) and the ipsilateral (ipsi) eye to the optimal spatial frequency. The phase disparity tuning functions were derived using optimal sinusoidal gratings (0.08 cycles deg⁻¹) at a temporal frequency of 4 Hz and a contrast of 30 %.

Figure 3. Examples of phase disparity tuning profiles for six binocular cells

The cell in A shows a phase insensitive profile while the others are phase sensitive. Cells in B, D, E and F show binocular facilitation while the cell in C shows binocular summation to phase disparity. Horizontal lines represent the monocular response of the ipsilateral (I) and the contralateral (C) eye at the optimal spatial frequency. The stimuli used to derive the phase tuning functions were optimal sinusoidal grating for the dominant eye (cell in A: 0.28 cycles deg⁻¹; cell in B: 0.14 cycles deg⁻¹; cell in C: 0.12 cycles deg⁻¹; cell in D: 0.56 cycles deg⁻¹; cell in E: 0.1 cycles deg⁻¹; cell in F: 0.1 cycles deg⁻¹) at a temporal frequency of 4 Hz; except for the cell in F, which was tested at 2 Hz. The contrast was 30 % (cells in A to D) or 50 % (cells in E and F). Cells in A, B and E were classified as complex while cells in C and F were classified as end-stopped complex. The cell in D was classed as an end-stopped simple.

Figure 4. Modulation index derived from the phase disparity tuning profiles

A, distribution of the modulation indices of 44 phase-sensitive cells. Cells having an index ≥ 30 are classified as phase sensitive. Most of the phase-sensitive cells have high modulation indices (mean = 74 %) ranging from 43 to 97 %. B, relationship between facilitation and the modulation indices of phase-sensitive neurons. The relationship between the facilitation index and the modulation index shows a positive correlation. Values located at and above the dashed line represent binocular facilitation interactions at the optimal phase disparity.

Figure 5. Relationship between displacement and spatial frequency of phase disparity-sensitive cells

A, distribution of the optimal displacement in visual angle of phase-sensitive cells. B, relationship between optimal displacement and optimal spatial frequency of the dominant eye. The relationship is demonstrated by the regression line and is negatively correlated. C, distribution of the spatial phase bandwidth of phase-sensitive cells. D, relationship between the spatial phase bandwidth and the optimal spatial frequency of the dominant eye. The relationship is negatively correlated, as shown by the regression line.