Andrew M. Derrington

Refraction, aliasing, and the absence of motion reversals in peripheral vision

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd.m-2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7-14 c/deg at 20 deg, 5.5-7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial.(ABSTRACT TRUNCATED AT 250 WORDS)

Feedback from V1 and inhibition from beyond the classical receptive field modulates the responses of neurons in the primate lateral geniculate nucleus

It is well established that the responses of neurons in the lateral geniculate nucleus (LGN) can be modulated by feedback from visual cortex, but it is still unclear how cortico-geniculate afferents regulate the flow of visual information to the cortex in the primate. Here we report the effects, on the gain of LGN neurons, of differentially stimulating the extraclassical receptive field, with feedback from the striate cortex intact or inactivated in the marmoset monkey, Callithrix jacchus. A horizontally oriented grating of optimal size, spatial frequency, and temporal frequency was presented to the classical receptive field. The grating varied in contrast (range: 0-1) from trial to trial, and was presented alone, or surrounded by a grating of the same or orthogonal orientation, contained within either a larger annular field, or flanks oriented either horizontally or vertically. V1 was ablated to inactivate cortico-geniculate feedback. The maximum firing rate of LGN neurons was greater with V1 intact, but was reduced by visually stimulating beyond the classical receptive field. Large horizontal or vertical annular gratings were most effective in reducing the maximum firing rate of LGN neurons. Magnocellular neurons were most susceptible to this inhibition from beyond the classical receptive field. Extraclassical inhibition was less effective with V1 ablated. We conclude that inhibition from beyond the classical receptive field reduces the excitatory influence of V1 in the LGN. The net balance between cortico-geniculate excitation and inhibition from beyond the classical receptive field is one mechanism by which signals relayed from the retina to V1 are controlled.

Motion of complex patterns is computed from the perceived motions of their components.

A plaid pattern made by adding two gratings of the same spatial frequency, one moving 45 deg above the horizontal, and the other moving 45 deg below the horizontal, appears to move horizontally when the speeds of the two components are equal. If the apparent speed of the upward-moving component is reduced by a motion after-effect (MAE), the plaid appears to move obliquely downwards, unless the actual speed of the downward-moving component is reduced to match the (reduced) apparent speed of the upward moving component. This is consistent with the hypothesis that the visual system computes the motion of a plaid pattern in two stages, first estimating the motions of the components, and then combining them according to the intersection of constraints. An alternative explanation: that the vertical component of the plaids motion is caused by an MAE in a horizontally oriented distortion product generated by non-linear transduction or transmission of the plaid, is ruled out by the finding that the adapting stimulus causes only a very weak vertical MAE.

Second-order motion discrimination by feature-tracking

When a plaid pattern (the sum of two high spatial frequency gratings oriented +/- 84 degrees from vertical) jumps horizontally by 3/8 of its spatial period its contrast envelope, a second-order pattern, moves in the opposite direction to its luminance waveform. Observers report that the pattern moves in the direction of the contrast envelope when the jumps are repeated at intervals of more than 125 ms and in the direction of the luminance profile when they are repeated at longer intervals. When a pedestal [Lu, Z.-L. & Sperling, G. (1995). Vision Research, 35, 2697-2722] is added to the moving plaid a higher contrast is required to see motion of the contrast envelope but not to see the motion of the luminance profile, suggesting that the motion of the contrast envelope is sensed by a mechanism that tracks features. Static plaids with different spatial parameters from the moving pattern are less effective at raising the contrast required to see the motion of the contrast envelope and simple gratings of low or high spatial frequency are almost completely ineffective, suggesting that the feature-tracking mechanism is selective for the type of pattern being tracked and rejects distortion products and zero-crossings.

Long-range interactions modulate the contrast gain in the lateral geniculate nucleus of cats

In previous work, we have shown that sudden image displacements well outside the classical receptive field modulate the visual sensitivity of LGN relay cells. Here we report the effect of image displacements on the response versus contrast function. The stimuli consisted of a central spot of optimal size and polarity (contrast range: 3-98%), flashed alone or in the presence of a peripheral annulus (radii: 5-15 deg) containing a low spatial-frequency grating displaced at saccade-like velocities (shift). The most consistent effect of the shift on the response to a central spot was to reduce the responsiveness of Y relay cells and, to a lesser extent, of X relay cells. The reduction in responsiveness was primarily a divisive rather than a subtractive effect and could be modelled by assuming that a greater contrast was required to produce a given excitatory response. In the absence of a central spot, remote motion had inhibitory effects on the firing rates of the majority of relay cells, but its effect on retinal ganglion cells was mainly excitatory. When the shifting grating covered the classical receptive field and its periphery, facilitatory effects or suppressive effects, depending on the spatial phase of the pattern, were observed in both retinal and geniculate cells. Remote motion strongly suppresses the responsiveness of relay cells to stimuli within the classical receptive field. This suppressive effect involves intrageniculate processing and is primarily associated with a reduction in contrast gain. It is likely that shift suppression contributes to the loss of visual sensitivity observed in saccadic suppression.

Temporal resolution of dichoptic and second-order motion mechanisms

We addressed the question of whether low-level motion analysers can integrate signals binocularly. We compared the temporal sensitivity in motion discrimination tasks using monocular and dichoptic first-order motion and monocular and dichoptic second-order motion. Three human observers were required to discriminate the direction of motion of either sinusoidal gratings (1 c/deg), used as a stimulus for first-order motion analysers, or the envelopes of contrast-modulated stationary sinusoidal gratings (carrier frequency 5 c/deg, carrier contrast 0.1, modulation frequency 1 c/deg), used as a stimulus for second-order motion analysers. Contrast sensitivity was measured as a function of temporal frequency. The moving grating or envelope was generated by summing two non-moving sinusoidally flickering gratings or envelopes in spatiotemporal quadrature. These were either combined monocularly or presented dichoptically. Sensitivity to the moving envelope was highest at a temporal frequency between 0.5 and 2 Hz, depending on the observer, and declined rapidly at high temporal frequencies. None of the observers was able to discriminate the direction of motion of envelopes moving faster than 4 Hz. Dichoptic and monocular presentation produced very similar results. Sensitivity to a monocularly presented moving grating was fairly uniform between 1 and 8 Hz, and declined slightly at 16 Hz. In one of three observers sensitivity to the dichoptically presented grating was very close to that of the monocularly presented grating at all temporal frequencies tested (from 1 to 16 Hz). All observers could discriminate the direction of motion of the dichoptically presented grating at 8 Hz, but two of the three were unable to discriminate its direction of motion at 16 Hz. These results indicate that second-order motion analysers have very poor temporal resolution and that dichoptic motion analysers have very good resolution. We suggest that this implies that there are low-level motion analysers that are capable of integrating information binocularly.

Second-order processes in vision: introduction

led to a radical reconceptualization ofthe nature of visual processing within the framework oflinear systems theory. Pattern adaptation and thresholdpattern-discrimination experiments provided dramaticevidence that human vision filters the retinal input into anumber of channels selectively tuned for spatiotemporalfrequency and orientation. These channels were concep-tualized as approximately linear, shift-invariant transfor-mations of the time-varying retinal input. It was hy-pothesized that these filters were realized byretinotopically organized arrays of neurons. The neu-rons in a given such array were all assumed to have thesame receptive field profile (typically modeled as a Gaborfunction of some specific spatial frequency and orienta-tion), and to monitor different locations in the stimulusfield: one neuron for each location. Thus the responsesof these neurons in a given channel array composed a‘‘neural image’’

Motion of contrast-modulated gratings is analysed by different mechanisms at low and at high contrasts

We used a pedestal test [Lu & Sperling (1995a). Vision Research, 35, 2697-2722] to determine whether motion discrimination of contrast-modulated gratings has different properties at low contrast (4.5%) and at high contrast (45%). The amplitude-modulated gratings consisted of a 5 c/deg static carrier modulated by a moving 1 c/deg contrast envelope. We found that when contrast is low direction discrimination for contrast-modulated gratings is vulnerable to pedestals and becomes impossible at about 4 Hz. At high contrast contrast-modulated gratings are unaffected by pedestals and modulation sensitivity in a motion direction-discrimination task remains high up to 12 Hz. These results are consistent with the hypothesis that separate mechanisms analyse motion of contrast-modulated gratings at low and at high contrast; at low contrast motion analysis is based on feature tracking, whereas at high contrast, contrast-modulated gratings are analysed by spatio-temporal filters.

Visual calibration of CRT monitors

Abstract In this paper, we develop and test a technique for calibrating a computer-controlled television monitor using a visual comparison instead of a photometer. The basic principle of the calibration is to compare a patch of pixels that are uniformly driven for an adjustable voltage with a patch in which a predetermined fraction of the pixels are set to the maximum voltage and the remainder are set to the minimum. By adjusting the voltage to make the two patches appear equally bright we get an estimate of the voltage that produces the predetermined fraction of the maximum luminance. Smooth functions were fit to the relationship between the DAC output and the fraction of illuminated pixels using a least-squares method, and used to estimate the function relating screen luminance to voltage. This function was then used to calculate lookup tables for linearisation. Sinusoidal and beat (sum of two sinusoids) luminance modulations were generated from the calibrated lookup tables and their profiles were measured with a photometer in order to check the calibrations. We find that visual calibration is sufficiently reliable to be used as an alternative to calibration using a photometer. It is easier and cheaper than using a photometer: a good photometer can be more expensive than the combined cost of the computer, graphics card and monitor.

Behaviour of marmoset monkeys in a T-maze: comparison with rats and macaque monkeys on a spatial delayed non-match to sample task

Abstract.The marmoset (Callithrix jacchus) is a small New World monkey that is increasingly being used in a laboratory setting. A previous set of studies has provided a direct comparison between the performance of rats and macaque monkeys on a spatial delayed non-match to sample task in a T-maze (Murray et al. 1989, Experimental Brain Research 74:173–186; Markowska et al. 1989, Experimental Brain Research 74:187–201). In the current experiment we replicated these studies using the marmoset. This allowed for a comparison of the behavioural performance of the marmoset with both rats and macaque monkeys. Marmosets performed well at the task, performing better than macaques, and at a similar level to rats. A closer analysis of the data from the present experiment suggests that marmosets spontaneously alternated in the T-maze, a strategy often adopted by rats, but not by macaques in the T-maze.