Binocular phase interactions in area 21a of the cat

Abstract

1. Binocular interactions related to retinal disparity were investigated in single neurons in area 21a of extrastriate cortex in the anaesthetized cat using sinusoidal luminance gratings. 2. The responses of approximately two-thirds of neurons were profoundly modulated by a relative phase difference between identical drifting gratings presented to each eye. This modulation included both facilitatory and inhibitory interocular interactions. The selectivity for binocular disparity was about twice as sharp as the selectivity for monocular spatial position. 3. Significant phase modulation was retained in many neurons at interocular orientation differences exceeding 45 deg. The response suppression associated with stimulation at a phase shift 180 deg from the optimum was stronger than the response suppression to an interocular orientation difference of 90 deg. 4. The proportion of phase modulated neurons and the potency of modulation in area 21a neurons exceed that reported for phase-selective complex cells in area 17. Neurons in area 21a show sharp disparity tuning that is relatively insensitive to changes in orientation and monocular position, which suggests that this extrastriate region has a role in stereoscopic depth perception.

Figure 1. Response modulation of two area 21a neurons to a relative phase offset between dichoptically presented drifting sinusoidal gratings

A, neuron showing strong facilitation and suppression. PSTHs accumulated over five runs are shown on the left; the mean ±s.d. response is plotted on the right, together with the monocular responses to optimum stimulation (dashed lines: C = contralateral eye, I = ipsilateral eye). The fitted curve is a sine function constrained to a period of one cycle. B, neuron with relative phase modulation typical of our sample.

Figure 2. Measures of disparity tuning in the sample of area 21a neurons

A, distribution of the phase modulation index for all neurons. ▪, neurons whose phase modulation was statistically significant (ANOVA, P < 0.05). , neurons whose phase modulation was not statistically significant. B, the BII is plotted against the phase modulation index for 46 neurons. ○, values with an S/N ratio less than 2 (see text).

Figure 3. Relation between binocular phase modulation and the optimum orientation and spatial frequency of area 21a neurons

A, distribution of BII plotted against the optimum orientation determined with stimulation of the dominant eye for all 46 cells. B, distribution of BII plotted against the optimum spatial frequency for the dominant eye. Two of the 46 cells were classified as spatial low-pass, and are plotted at 0.02 cycles (c) deg⁻¹.

Figure 4. Facilitation and inhibition of area 21a neuron responses by binocular phase interaction

The maximum binocular response (abscissa) and the minimum binocular response (ordinate) are expressed as the change from the monocular dominant eye response level.

Figure 5. Relation between phase modulation and interocular orientation

A, responses of an area 21a neuron plotted against the difference in interocular orientation. •, responses at the optimum relative phase offset at each interocular orientation; ○, responses at a 180 deg phase shift from the optimum. The dotted line shows the monocular dominant eye response at the optimum orientation. B, phase modulation for 36 neurons is plotted as the difference between the response at the optimum phase and at a phase shift of 180 deg from optimum against the difference between the response to the iso-oriented and to the orthogonal gratings at the optimum phase for 36 neurons. C, mean ±s.e.m. BII of nine neurons at interocular orientation differences ranging from 0 to 90 deg. The dashed line indicates a BII level of 0.3, which is the criterion level adopted to indicate disparity selectivity.

Figure 6. Relation between monocular and binocular relative modulation for the 28 neurons that had binocular phase responses well described by a sine function, and a monocular response exceeding 5 impulses s⁻¹

The binocular relative modulation is twice the BII; the monocular relative modulation is the f₁/f₀ ratio (see Results). Cells with a monocular relative modulation greater than 1 were classified as simple-like cells (shaded area). The 45 deg line indicates expected performance for well-matched monocular and binocular modulation: this holds true for simple-like cells, but not for complex-like cells.