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H B Barlow

Publications and source records attributed to H B Barlow.

13 recordsLinked to original sources
PubMed1987

Limit to the detection of Glass patterns in the presence of noise.

A method is developed for quantifying the strength of the moiré effects known as Glass patterns. Unpaired randomly placed dots are added to the pattern while the discriminability d' of the degraded pattern is determined in a yes-no test. For a given discriminability the number of pairs required increases in direct proportion to the number of random dots. A model is developed based on the ideal discrimination of an excess of oriented pairs. Results conforming to Weber's law are predicted; the dependence of d' on the amount of noise and the number of point pairs is also predicted. A numerical constant derived from the model provides a measure of the strength of the moiré effect of a chosen pattern. Note that, for this task, statistical considerations predict Weber's law, not the square-root law, as a limit, and this result holds whenever second-order structure is detected in the image.

Form Perception↗
PubMed1987

Human contrast discrimination and the threshold of cortical neurons.

The human contrast-discrimination function has a curious shape: In addition to rising for increasing contrasts, both positive and negative, it also rises for very low contrasts on either side of zero. It is shown that this rise near zero contrast is not much affected by procedures that increase or decrease the subject's knowledge of the stimulus; this counts against uncertainty as the immediate cause of the elevation near zero contrast. The alternative explanation in terms of a genuine response threshold is shown to be promising when measurements of human contrast discrimination are compared with values calculated from records of neurons in monkey primary visual cortex. The comparison also suggests that the dynamic range of single neurons is lower than that shown psychophysically. It is suggested that having a cortical threshold may be the visual system's way of preventing false positives when there is much stimulus uncertainty. This threshold may also help to explain why quantum efficiencies calculated from detection thresholds are so poor compared with those estimated from the visual system's susceptibility to added noise.

Humans↗
PubMed1977

The effects of remote retinal stimulation on the responses of cat retinal ganglion cells.

1. Action potentials were recorded from optic nerve fibres of lightly anaesthetized cats while parts of the retina remote from the receptive field were stimulated by a shifting grating. 2. Vigorous responses can be obtained under these conditions, confirming McIlwain (1966), Krüger & Fischer (1973), and others. 3. These 'shift responses' are not caused by fluctuations of stray light because (a) they cannot be reduced by deliberately increasing or decreasing the light falling on the receptive field synchronously with the shifting grating; (b) a steady adapting light applied to the receptive field does not raise the threshold for the responses, whereas adapting light on the peripheral retina does, and (c) the threshold for the responses is elevated more following bleaching adaptation of the periphery than following bleaching adaptation of the centre. 4. Shift responses are strong, of short latency, and brief in duration in brisk-transient (Y-type) neurones. With few exceptions they are weak but long-lasting in brisk-sustained (X-type) neurones. 5. Shift responses are unlike responses from the main receptive field in having a distinct threshold; the magnitude of the response to weak gratings is not simply proportional to contrast, as is the case with weak stimuli applied to the receptive field. 6. It is thought that the excitatory pathway may involve amacrine cells, and that this mechanism may be concerned with the detection of the shifts of the image that occur with saccadic eye movements.

Action Potentials↗
PubMed1976

Threshold setting by the surround of cat retinal ganglion cells.

1. The slope of curves relating the log increment threshold to log background luminance in cat retinal ganglion cells is affected by the area and duration of the test stimulus, as it is in human pyschophysical experiments. 2. Using large area, long duration stimuli the slopes average 0-82 and approach close to 1 (Weber's Law) in the steepest cases. Small stimuli gave an average of 0-53 for on-centre units using brief stimuli, and 0-56 for off-centre units, using long stimuli. Slopes under 0-5 (square root law) were not found over an extended range of luminances. 3. On individual units the slope was generally greater for larger and longer test stimulus, but no unit showed the full extent of change from slope of 0-5 to slope of 1. 4. The above differences hold for objective measures of quantum/spike ratio, as well as for thresholds either judged by ear or assessed by calculation. 5. The steeper slope of the curves for large area, long duration test stimuli compared with small, long duration stimuli, is associated with the increased effectiveness of antagonism from the surround at high backgrounds. This change may be less pronounced in off-centre units, one of which (probably transient Y-type) showed no difference of slope, and gave parallel area-threshold curves at widely separated background luminances, confirming the importance of differential surround effectiveness in changing the slope of the curves. 6. In on-centre units, the increased relative effectiveness of the surround is associated with the part of the raised background light that falls on the receptive field centre. 7. It is suggested that the variable surround functions as a zero-offset control that sets the threshold excitation required for generating impulses, and that this is separate from gain-setting adaptive mechanisms. This may be how ganglion cells maintain high incremental sensitivity in spite of a strong maintained excitatory drive that would otherwise cause compressive response non-linearities.

Action Potentials↗