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Biomedical subjects

D C Burr

Publications and source records attributed to D C Burr.

69 records · Page 4Linked to original sources
PubMed1985

Local regulation of luminance gain.

Contrast sensitivity was measured for sinusoidal gratings sampled by compressing luminance into a variable number of sample bars. This procedure does not affect the amplitude or mean luminance of the grating, but does increase the local luminance of the sample bars: the fewer the bars, the more luminous they are. It was found that sensitivity increased with bars per cycle, particularly at low spatial frequencies. Further experiments, in which the local luminance of the sampling bars (but not the average luminance of the grating) was varied by addition of veiling glare showed that contrast sensitivity varied inversely with local bar luminance (a Weber type relationship). We interpret the results as evidence of local gain control under conditions where average luminance, and hence mean photon flux, does not vary. Calculations based on variation of sensitivity with spatial frequency suggest that gain control can be very localized, with receptive fields of Gaussian space constant of 0.5' arc. The relevance of these results to modern psychophysical concepts, including the definition of contrast is discussed.

Form Perception↗
PubMed1984

Summation of target and mask metacontrast stimuli.

Three experiments are reported; they show the following: (i) Metacontrast occurs at high photopic conditions (250 cd m-2); at this luminance the function peaks at stimulus asynchronies of approximately 40 ms (compared with 100 ms at lower luminances). (ii) The strength of metacontrast is contrast-independent, down to contrast very near detection threshold. (iii) Detection thresholds for the combined target and mask presentation are lower than those for the mask alone, implying summation between target and mask. Summation is greatest at the same asynchronies which yield maximum metacontrast masking. The experiments are taken as evidence in favour of a fusion explanation for metacontrast. An argument is made which implicates temporally units of the motion detection system as being instrumental in the fusion process.

Humans↗
PubMed1982

Functional implications of cross-orientation inhibition of cortical visual cells. I. Neurophysiological evidence.

Simple and complex cells of striate cortex of anaesthetized and paralysed cats were stimulated with two superimposed one-dimensional grating stimuli of different orientations to investigate inhibitory effects of non-optimally oriented stimuli. We confirmed that a stimulus of orientation orthogonal to a cell's long axis significantly reduces the cell's discharge rate. Further experiments revealed the following. (i) The inhibition was typically stronger for simple than for complex cells. (ii) It is very broadly tuned for orientation, all orientations outside the cell's tuning band having a comparable inhibitory effect. (iii) Similarly, it is broadly tuned for spatial frequency. These last two results suggest that the inhibition arises not from a single cell but from a pool of cells. (iv) The pattern of the discharge of the inhibition in response to stimulation by phase-reversed sinusoidal gratings is consistent with the notion that the inhibition arises from complex cells. A second series of recordings of stimulation by visual noise patterns demonstrated how 'cross-orientation inhibition' prevents simple cells from responding to two-dimensional visual noise while allowing them to respond to comparable one-dimensional noise patterns. We suggest that this mechanism may serve to render simple cells selectively sensitive to one-dimensional stimuli, such as the contours or borders of visual objects.

Action Potentials↗
PubMed1982

Contrast sensitivity at high velocities.

Measurements were made of the contrast required to see the direction of motion of drifting gratings (Part 1) and of moving bars (Part 2). The spatial frequency at which least contrast is required to see sinusoidal gratings decreases as their velocity increases, but peak sensitivity is identical at all velocities up to 800 deg/sec. Similarly, the wider a single bar, the higher the velocity at which it is best visible. A bar 80 deg wide is best seen when moving at 300-500 deg/sec, and can be seen, and its direction of motion identified, even when moving at 10(4) deg/sec. These results show that motion does not diminish the visual passband, but instead slides the spatial frequency window along the spatial frequency scale, maintaining peak sensitivity at a temporal frequency of about 10 Hz (at photopic luminances).

Differential Threshold↗
PubMed1982

Selective depression of motion sensitivity during saccades.

1. Horizontal gratings flashed for 20 ms were used to compare visual contrast sensitivity during horizontal saccades with sensitivity during normal vision, at three luminance levels, 4 X 10(2), 4 X 10(-2), and 4 X 10(-4) cd/m2. 2. Greatest sensitivity loss during saccades was found at low spatial frequencies. There is little or no loss at high spatial frequencies. 3. As luminance level is decreased there is a decrease in the spatial frequency below which saccadic sensitivity loss occurs. This shift in spatial frequency with luminance level, considered in conjunction with measurements of stationary and drifting gratings, indicates the functional involvement of movement sensitive mechanisms in saccadic sensitivity loss. 4. At the two lower luminance levels (4 X 10(-2) and 4 X 10(-4) cd/m2) sensitivity during saccades is greater than normal at high spatial frequencies. This enhancement of sensitivity was confirmed by forced choice measurements. 5. It was also shown that sensitivity to abrupt changes in the trajectory of moving gratings is lowered during saccades. 6. It is concluded that mechanisms sensitive to movement and transients are damped during saccades, so preventing perception of image motion during saccades and thereby preserving visual stability.

Eye Movements↗
PubMed1981

Temporal summation of moving images by the human visual system.

Measurements of threshold visibility were made as a function of duration of stimulus exposure for small moving dot targets, drifting sinusoidal gratings and moving patches of sinusoidal gratings, to investigate how the human visual nervous system summates over time signals arising from stimuli in motion. At image speeds of less that 16 deg/s, temporal summation is as strong and as extended for moving as for stationary dots (total summation over to about 100 ms). This summation is about twice that which would be expected from separate consideration of the regions of spatial and temporal integration. Measurements with sinusoidal gratings reveal that the nature of the summation depends critically on the spatial frequency of the stimulus: gratings of low spatial frequency summate well when in motion (and only when in motion), whereas those of high spatial frequency summate well only when stationary or in very slow motion. An analogue simulation with electronic filters showed that these psychophysical results are directly predictable from the known transfer characteristics of the human visual system (with the additional assumption of probability summation at threshold). Finally, with small patches of sinusoidal grating, it was established that translation per se across the retina has little effect on temporal summation. This suggests that the results obtained with sinusoidal gratings of large extent are also relevant to small moving stimuli, allowing the summation results obtained with dot stimuli to be discussed in terms of the temporal transfer properties of spatially selective visual detectors. On the basis of these results it is proposed that the extended temporal summation observed for dots in motion results from summation of energy of low spatial frequency present in these stimuli.

Computers↗
PubMedDate not supplied

Pattern-reversal electroretinogram in response to chromatic stimuli: I. Humans.

We have studied the steady-state PERG in human subjects in response to red-green plaid patterns modulated either in luminance or in chromaticity or both. By varying the relative luminance of the red and green components, a value could be obtained at which the PERG amplitude was either minimum or locally maximum. This always occurred at equiluminance, as measured by standard psychophysical techniques. PERG amplitude and phase were measured as a function of spatial and temporal frequency of sinusoidal contrast reversal. In both space and time, the response to chromatic patterns was low-pass, while that to luminance was band-pass, and extended to higher spatial and temporal frequencies. The phase of the PERG to chromatic stimuli was systematically lagged compared with that to luminance stimuli, by an amount corresponding to about 20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 67 ms for color contrast compared with 47 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. For both luminance and chromatic stimuli, the amplitude of PERGs increases with increasing stimulus contrast. By summing vectorially the responses to appropriate luminance and chromatic contrasts, we were able to predict with accuracy the response as a function of color ratio (ratio of red to total luminance). The above findings all agree with those reported in the accompanying paper for the monkey PERG (Morrone et al., 1994), and indicate that the differences in response latency and integration time of luminance and chromatic stimuli observed by psychophysical and VEP techniques may arise at least in part from the properties of retinal mechanisms.

Color Perception↗
PubMedDate not supplied

Pattern-reversal electroretinogram in response to chromatic stimuli: II. Monkey.

We have recorded steady-state PERGs from five macaque monkeys in response to red-green plaid patterns reversed sinusoidally in contrast. The patterns had either a pure luminance contrast (red-black, green-black, yellow-black), pure red-green color contrast, or a variable amount of luminance and color contrast. By varying the relative luminance of the red-to-total luminance (color ratio) of red-green patterns, a value could be obtained at which the PERG amplitude was either minimum or locally maximum, and the phase was most lagged. This value was very similar to that producing equiluminance in human observers, and was considered to be equiluminance for the monkey. The phase of the PERG to chromatic stimulus was systematically lagged compared with that of luminance stimuli, by an amount corresponding to about 10-20 ms under our experimental conditions. The variation of phase with temporal frequency suggested an apparent latency of about 80 ms for color contrast compared with 63 ms for luminance. These estimates were confirmed with separate measurements of transient PERGs to abrupt contrast reversal. As a function of temporal frequency, the chromatic PERG function was clearly low-pass with a cutoff around 15 Hz, whereas that to luminance was double-peaked and extended to higher temporal frequencies, around 30 Hz. For both luminance and chromatic stimuli, the amplitude of PERGs increases with increasing stimulus contrast. By summing vectorially the luminance and chromatic responses of appropriate contrasts, we were able to predict with accuracy the response as a function of color ratio. In two monkeys, the optic chiasm was sectioned sagittally causing total degeneration of ganglion cells in the nasal retina, without affecting the temporal retina (verified by histology). In these animals, there was a strong response to both luminance and chromatic patterns in the temporal retinae, but none to either type of pattern in the nasal retinae, suggesting that the PERG to both luminance and chromatic stimuli arises from the inner-retinal layers. Electrophysiological studies suggest that the PERG to chromatic stimuli is probably associated with the activity of P-cells. P-cells may also make a major contribution to the PERG of luminance stimuli, although M-cells may also participate. The above findings on normal monkeys all agree with those reported in the accompanying paper for humans (Morrone et al., 1994), so similar conclusions can probably be extended to human PERG.

Animals↗
PubMedDate not supplied

Spatial neglect is associated with increased latencies of visual evoked potentials.

We have recorded steady-state visual evoked potentials (VEPs) from patients with vascular damage to their right brain hemispheres, some suffering from unilateral spatial neglect (n = 9), and some not (n = 7). VEPs were recorded in response to sinusoidal gratings of 0.56 cycle/deg contrast-reversed sinusoidally at temporal frequencies from 4-11 Hz. Stimuli were presented either to the left or to the right visual field, or to both. Confirming previous reports, reliable VEPs were recorded from stimuli in the left contralesional hemifield, of comparable amplitude to those of the ipsilesional hemifield and to those of both hemifields of brain damaged patients without neglect. However, analysis of apparent latency derived from phase data showed that the VEPs from the contralesional hemifield were systematically delayed by 30-40 ms compared with those of the ipsilesional hemifield, and compared with both hemifields of the nonneglect groups. This result suggests changes in neural processing in neglect patients.

Adult↗
PubMedDate not supplied

Added noise restores recognizability of coarse quantized images.

When a portrait is coarsely quantized into blocks, the block structure hides the face, although lower spatial frequencies of the original image sufficient by themselves for recognition are preserved. Recognition can be recovered by blurring the image, or otherwise attenuating the spurious higher spatial frequency components. Harmon and Julesz claim that high spatial frequencies introduced by quantized blocking mask the lower spatial frequencies which convey information about the face, preventing recognition. Here we show that recognition can be enhanced, without decreasing the amplitude of these spurious higher frequencies, by adding further high-frequency noise to the quantized image. This result is clearly at odds with a theory of high-frequency (or critical band) masking. We suggest that the added noise mutes mechanisms which would otherwise impose a block structure on the image, allowing the alternative perceptual organization of the hidden face to reemerge.

Form Perception↗
PubMedDate not supplied

Evidence for the existence and development of visual inhibition in humans.

Neural inhibition forms a major mechanism by which the nervous system refines and elaborates its input. Several recent experiments have demonstrated the existence of inhibition between orientation-selective cells of the primary visual cortex of the cat and although the precise function of this inhibition is uncertain, there is evidence that it enhances orientation tuning and that it is involved in pattern recognition. Here we report a series of experiments which, on the basis of evoked potential responses to oriented stimuli, suggest that similar processes may exist in humans. Recordings from young infants further suggest that the machinery which mediates orientation-specific interactions may not be functional at birth, but develops only after 6-8 months.

Adult↗