Electrophysiological correlates of visual perceptual masking in monkeys

Summary Monkeys were trained to discriminate with nearly 100% accuracy between a square and a triangle presented simultaneously in a brief tachistoscopic flash. Perceptual masking was demonstrated by inability to perform this trained visual discrimination at better than chance level when the information flash was followed in less than 20 msec by a blank second flash. In order to determine the nature and locus of this retroactive visual perceptual masking effect, electrical potentials were recorded simultaneously from three points along the optic pathways: optic nerve or tract, lateral geniculate body, and visual cortex. Potentials were computer-averaged and correlated with the monkey's ability or inability to make a correct behavioral response to the information contained in the first or test flash (T). When the perception of T was masked by the second or blanking flash (B), only the evoked potential characteristic of B was observed at all recording sites, including the optic nerve or tract. This suggested that the interaction underlying masking occurred in the retina since optic nerve or tract responses are dependent upon retinal ganglion cells. When T was not masked, an early portion of the evoked response to T could be detected at all recording sites. Perception of T was possible when only the initial segment of the T-potential (15 msec or less) was present at each recording site. Thus the visual information transfer essential to the performance of an already learned visual discrimination task appears to occur very early in the course of the evoked response and is not dependent upon later or secondary components.Supported by grants from the Office of Naval Research (Nonr-4756) and National Aeronautics and Space Administration (NASA NGL 05-007-049). Aided by National Institute of Mental Health Training Grant 5 TI MH-6415.     

Background configuration and rod threshold

1. This paper investigates the variation in rod threshold when a small test flash is seen against backgrounds of different sizes. Over a substantial range of luminances above absolute threshold, the test flash is less easily seen against small backgrounds than large. This confirms earlier results.2. If an annular surround is added to a small circular background, threshold is reduced when background and annulus are equiluminous (uniform field), but rises rapidly as the annulus is made brighter or dimmer than the background. This cannot be explained by the threshold-elevating effects of light scattered on to the background from the surround, for threshold rises with annulus luminance faster than it does on uniform fields of equal luminance.3. If the surround is not a complete annulus but a windmill-shaped cross, threshold is higher than on a uniform field, no matter what the windmill luminance. Thus it is not the addition of light per se to the surround which reduces threshold.4. This conclusion is reinforced by the results of another experiment. The test flash is seen on a large uniform field. When superimposed on this field, a thin ring, light or dark, which causes only a small change in mean luminance, produces an appreciable rise in threshold.5. The addition of an equiluminous red surround to a small red background so as to create a uniform field causes a marked drop in test flash threshold, but a scotopically equal blue surround, that creates a uniform field for rods, does not alter the threshold. Since the test flash is seen only by rods it follows that signals from cones can alter rod threshold.6. Known or probable behaviour of retinal mechanisms cannot account for our results. All the operations which elevate threshold above its level on a large uniform field produce contours in the vicinity of the test flash. This we take as evidence that signals from the test stimulus are suppressed or reduced by other signals present only when the background is locally non-uniform.     

Orientational selectivity of the human visual system

1. It is known that an object is less detectable when it is viewed against a background containing structures similar to the object. The effect of changing the orientation between the object and background is investigated.

2. Gratings of variable contrast were generated on two oscilloscopes; these were superimposed optically. The angle of orientation between them could be changed. The threshold of one grating, the test grating, was determined in the presence of the other, the masking grating.

3. When the gratings were presented with the same orientation (and locked in phase) the increment threshold of the test grating was found to be proportional to the suprathreshold contrast of the masking grating.

4. As the angle between the test and masking gratings was increased the masking effect fell exponentially.

5. At 12° on either side of a vertical test grating the masking effect was reduced by a factor of two with respect to its maximum value. This angle was independent of the contrast level of masking, the focus, and also the phase coherence of the masking grating.

6. If the test grating was presented obliquely the effect of masking was slightly less.

7. The narrow orientationally tuned channels found psychophysically by this masking technique are compared with the orientationally sensitive cells discovered electrophysiologically in the visual cortex of the cat.

     

Suppression on neuronal responses by a metacontrast masking stimulus in monkey V4

We studied the temporal characteristics of suppression in area V4 of the monkey using a visual stimulus for metacontrast masking. Visual responses of V4 neurons to a brief test stimulus presented within the receptive field were recorded, and the effect of a mask stimulus that did not spatially overlap the test stimulus was examined. Responses to the test stimulus were suppressed by the mask stimulus, which either preceded or followed the test stimulus. To study the temporal characteristics of suppression, the interval between the onset of the test stimulus and that of the mask stimulus (stimulus onset asynchrony, SOA) was varied. Maximum suppression occurred with a simultaneous presentation of the two stimuli, and the suppression gradually weakened as the SOA increased. The suppressive effect of the mask stimulus lasted on average about 77 ms in the negative SOA (forward masking) and 65 ms in the positive SOA (backward masking). These results indicate that surround suppression in V4 neurons has considerable temporal width, which is longer than that previously reported in areas V1 and V2. There were marked differences between the time course of suppression in V4 neurons in the present study and those reported in human metacontrast masking.     

Signals from cones

1. We have studied red and green cones by contrast flash inhibition and found them both to be very similar to rods in their response to flash energy, except that all light quantities must be some 100-fold greater in cones for the same effect.2. Using the methods of a previous paper (A.R.T. a) where no backgrounds were employed, we plotted log test flash lambda against log surround flash varphi with criterion that lambda should just be detected. The experiment was repeated with a ;windmill stop' interposed in the varphi flash which reduced its area symmetrically to (1/8). From these two curves it is possible to extract the relation between N, the inhibitory signal, and varphi, the test flash. It isN = varphi/(varphi+sigma),where sigma, the semi-saturation constant, is about 4.5 log td sec.3. Using the methods of (A.R.T. b) where backgrounds were studied, we measured the increment threshold for the surround flash varphi against its background theta using as criterion not that varphi should just be seen but that it should generate a fixed inhibitory signal N so that the fixed test flash lambda could just be seen.4. This increment threshold curve resembled the Aguilar & Stiles (1954) curve for rods, showed saturation and a complete symmetry about the 45 degrees line through the point with co-ordinates (log theta(D), log sigma).5. These results imply that the cone signal N is related to flash varphi and steady background theta by [Formula: see text], where theta(D) is receptor noise (= eigengrau), and varphi and theta are expressed in units of quantum catch.6. The ordinary increment threshold for cones does not show saturation because a steady saturating background bleaches all the pigment away. When the background is presented for only 100 msec with dark pauses between, no great bleaching occurs and saturation is seen.     

Transcranial magnetic stimulation in the visual system. I. The psychophysics of visual suppression

When applied over the occipital pole, transcranial magnetic stimulation (TMS) disrupts visual perception and induces phosphenes. Both the underlying mechanisms and the brain structures involved are still unclear. The first part of the study characterizes the suppressive effect of TMS by psychophysical methods. Luminance increment thresholds for orientation discrimination were determined in four subjects using an adaptive staircase procedure. Coil position was controlled with a stereotactic positioning device. Threshold values were modulated by TMS, reaching a maximum effect at a stimulus onset asynchrony (SOA) of approx. 100 ms after visual target presentation. Stronger TMS pulses increased the maximum threshold while decreasing the SOA producing the maximum effect. Slopes of the psychometric function were flattened with TMS masking by a factor of 2, compared to control experiments in the absence of TMS. No change in steepness was observed in experiments using a light flash as the mask instead of TMS. Together with the finding that at higher TMS intensities, threshold elevation occurs even with shorter SOAs, this suggests lasting inhibitory processes as masking mechanisms, contradicting the assumption that the phosphene as excitatory equivalent causes masking. In the companion contribution to this one we present perimetric measurements and phosphene forms as a function of the stimulation site in the brain and discuss the putative generator structures.     

The nature of rise in threshold produced by contrast-flashes

1. The rod threshold for seeing a flash on a 2(1/2) degrees square is raised by a nearly simultaneous flash that falls on the surround. When this ;contrast-flash' is held fixed in intensity, it raises the log test threshold by a fixed amount no matter how far that threshold has already been raised by light adaptation owing to background or bleaching.2. This is surprising since fixed backgrounds and bleachings raise the log test threshold much more when the eye is dark than when light adapted.3. When the test flash is held at some fixed supra-threshold value, the contrast flash exhibits a ;critical level', above which the test will no longer be seen. If the surround region upon which the contrast-flash falls is adapted by background or bleaching, its efficacy is reduced so that the ;critical level' is raised.4. Surround adaptation raises the log ;critical level' by the same amount that it raises the log threshold for seeing the contrast-flash itself.5. The way that contrast flashes raise the test threshold is thus entirely different from the way that adaptations by bleachings or backgrounds do. Contrast-flash signals appear to inhibit test-flash signals by interaction at some point central to the site where adaptation occurs.6. This permits the effect of adaptation on signals to be measured. A given state of adaptation attenuates all flash signals in the same proportion. And in any state of adaptation a single flash will reach threshold when the attenuated signal has a fixed size.     

Central inhibitory interactions in human vision

Summary Contrast threshold and perceived orientation of a line segment were measured when another line segment was simultaneously presented either to the same or the other eye; the angle between the two line segments was varied. The presence of the masking line elevated the contrast threshold under both conditions and the threshold increased similarly both in monoptic and dichoptic masking when the masking angle was made smaller. The presence of the masking line affected also the perceived orientation of the test line. The effect was similar both in monoptic and dichoptic masking; the largest change of perceived orientation occurred at about 15° masking angle and the change was smaller at other angles. The effect disappeared within a short distance when the masking line was removed farther from the test line.The similarity of the monoptic and dichoptic threshold elevations demonstrates that there are lateral inhibitory interactions between central neural units in the human visual system. It is likely that the interacting units mediate the perception of contour orientation, for the threshold elevation functions were consistent with concurrent changes of perceived orientation. The results are evidence for the hypothesis that inhibition between orientation detectors is a factor in the perceptual expansion of acute angles.     

Motion-induced positional biases in the flash-lag configuration

When both stationary and moving objects are present in the visual field, localizing objects in space may become difficult, as shown by illusory phenomena such as the Fr?hlich effect and the flash-lag effect. Despite the efforts to decipher how motion and position information are combined to form a coherent visual representation, a unitary picture is still lacking. In the flash-lag effect, a flash presented in alignment with a moving stimulus is perceived to lag behind it. We investigated whether this relative spatial localization (i.e., judging the position of the flash relative to that of the moving stimulus) is the result of a linear combination of two absolute localization mechanisms--that is, the coding of the flash position in space and the coding of the position of the moving stimulus in space. In three experiments we showed that (a) the flash is perceived to be shifted in the direction of motion; (b) the moving stimulus is perceived to be ahead of its physical position, the forward shift being larger than that of the flash; (c) the linear combination of these two shifts is quantitatively equivalent to the flash-lag effect, which was measured independently. The results are discussed in relation to perceptual and motor localization mechanisms.     

Motion-induced positional biases in the flash-lag configuration

When both stationary and moving objects are present in the visual field, localizing objects in space may become difficult, as shown by illusory phenomena such as the Fröhlich effect and the flash-lag effect. Despite the efforts to decipher how motion and position information are combined to form a coherent visual representation, a unitary picture is still lacking. In the flash-lag effect, a flash presented in alignment with a moving stimulus is perceived to lag behind it. We investigated whether this relative spatial localization (i.e., judging the position of the flash relative to that of the moving stimulus) is the result of a linear combination of two absolute localization mechanisms—that is, the coding of the flash position in space and the coding of the position of the moving stimulus in space. In three experiments we showed that (a) the flash is perceived to be shifted in the direction of motion; (b) the moving stimulus is perceived to be ahead of its physical position, the forward shift being larger than that of the flash; (c) the linear combination of these two shifts is quantitatively equivalent to the flash-lag effect, which was measured independently. The results are discussed in relation to perceptual and motor localization mechanisms.     

Cone signals in the cat''s retina

1. The discharges of ganglion cells in the cat's retina were recorded under conditions intended to isolate the cone system.

2. Stiles' two-colour threshold technique permitted the photopic system to be studied when at its highest sensitivity. The absolute sensitivity of a ganglion cell, expressed in equivalent photons of λ_max at the cornea per impulse discharged, was about 2500 times less when driven by cones than when driven by rods. This ratio improves to around 200 when allowance is made for the much smaller fraction absorbed by cones of photons incident on the cornea.

3. The number of extra impulses discharged in response to a brief flash was approximately proportional to the number of photons in the flash, up to a limit.

4. There was a region in the middle of the receptive field within which the area of a test spot and its illumination for threshold varied inversely. A flash extending over the peripheral part of the receptive field raised threshold above its minimum, presumably as a result of surround antagonism. Assessed from area—threshold curves, the balance of centre-surround antagonism in the photopic receptive field did not seem to depend upon background illumination.

5. The threshold for a small (0·2°) flash confined to the middle of the receptive field was independent of background illumination until the background exceeded a particular level, the `dark light' (I_o). In different units this ranged about a mean of 7·89 log photons (560 nm equivalent) deg^-2 sec^-1. For backgrounds that exceeded I_o, threshold followed approximately Weber's law up to the highest illuminations that could be produced.

6. With test flashes that filled the centre of the receptive field, the Weber fraction (test flash illumination/background illumination) in some units fell below 1%.

7. Changes in the time course and latency of response accompanied the changes in sensitivity caused by alterations in background illumination. Responses of both X- and Y-cells became more transient and faster.

8. The loss of sensitivity to a test flash brought about by a steady background light depended upon the size of that light. Sensitivity varied inversely with background area within a central region that matched closely the summing area for test flashes.

     

Vibrotactile masking through the body

Touches on one hand or forearm can affect tactile sensitivity at contralateral locations on the opposite side of the body. These interactions suggest an intimate connection between the two sides of the body. Here, we explore the effect of masking not across the body but through the body by measuring the effect of a masking stimulus on the back on the tactile sensitivity of the corresponding point on the front. Tactile sensitivity was measured on each side of the stomach, while vibrotactile masking stimulation was applied to one side of the front and to points on the back including the point directly behind the test point on the front. Results were compared to sensitivity, while vibrotactile stimulation was applied to a control site on the shoulder. A reduction in sensitivity of about .8 dB was found that required the masking stimulus to be within about 2 cm of the corresponding point on the back.     

Cutaneous masking. I. Psychophysical observations on interactions of multipoint stimuli in man

1. Psychophysical masking of cutaneous sensation at the locus of punctate test stimulation has been quantitatively examined with phasic mechanical and brief air-pulse stimuli using a conditioning-test stimulus paradigm. 2. Masking was maximal at the minimal interstimulus distance effective with this paradigm, varies inversely with interstimulus distance, and is demonstrable with the conditioning and test stimuli up to 10 cm apart on the forearm. 3. The degree of masking was found to be a direct function of the relative intensity of the conditioning stimulus with respect to the test stimulus. 4. Variations in the interstimulus interval permitted an investigation of the temporal features of cutaneous masking. It was detectable from 10 ms before to 70 ms after conditioning stimulation. Maximum masking occurred when the test stimulus was delivered about 10 ms following conditioning stimulus onset. 5. We also noted the much less marked, but still significant, enhancement phenomenon, in which weak conditioning stimuli, at just-threshold intensity levels, lowered the detection threshold for sensation at the test stimulus locus. We found this enhancement of sensation to have the same spatial distribution as did masking, but a much reduced time course. It began with the test stimulus presented simultaneously with the conditioning stimulus, peaked with 10--15 ms interstimulus intervals, but decayed in less than 40 ms. 6. Since psychophysical experiments often form the framework for the understanding of physiologic processes, it is suggested that these behavioral determinations of enhancement and masking may be correlated with the electrophysiologic properties of excitation and inhibition in neurons of the major primary somatic pathways of the central nervous system.     

心理干预对耳鸣患者疗效的影响

目的探讨心理干预在耳鸣患者治疗中的效果，为临床应用提供依据。方法在耳鼻喉科门诊随机抽出以耳鸣为第一主诉患者121例，将患者随机分为两组，A组62例接受心理干预、掩蔽及药物治疗，B组59例接受掩蔽、药物治疗。治疗前后用SCL-90及耳鸣程度主观评估进行比较。结果两组患者治疗前SCL-90评分比较无差异（P〉0．05），治疗组治疗前后SCL-90评分在躯体化、人际关系敏感、抑郁、焦虑、敌对、恐怖、偏执因子上前后差异较为明显（P〈0．05，P〈0．01），对照组治疗前后各因子分无差异（P〉0．05），两组治疗后在躯体化、抑郁、焦虑、敌对、恐怖、偏执因子上相比均有显著差异（P〈0．05或P〈0．01）。两组患者疗效均有改善，而治疗组改善更为明显（P〈0．05）。结论心理干预对耳鸣患者有较好的疗效。     

Auditory‐visual integration modulates location‐specific repetition suppression of auditory responses

Space is a dimension shared by different modalities, but at what stage spatial encoding is affected by multisensory processes is unclear. Early studies observed attenuation of N1/P2 auditory evoked responses following repetition of sounds from the same location. Here, we asked whether this effect is modulated by audiovisual interactions. In two experiments, using a repetition‐suppression paradigm, we presented pairs of tones in free field, where the test stimulus was a tone presented at a fixed lateral location. Experiment 1 established a neural index of auditory spatial sensitivity, by comparing the degree of attenuation of the response to test stimuli when they were preceded by an adapter sound at the same location versus 30° or 60° away. We found that the degree of attenuation at the P2 latency was inversely related to the spatial distance between the test stimulus and the adapter stimulus. In Experiment 2, the adapter stimulus was a tone presented from the same location or a more medial location than the test stimulus. The adapter stimulus was accompanied by a simultaneous flash displayed orthogonally from one of the two locations. Sound‐flash incongruence reduced accuracy in a same‐different location discrimination task (i.e., the ventriloquism effect) and reduced the location‐specific repetition‐suppression at the P2 latency. Importantly, this multisensory effect included topographic modulations, indicative of changes in the relative contribution of underlying sources across conditions. Our findings suggest that the auditory response at the P2 latency is affected by spatially selective brain activity, which is affected crossmodally by visual information.     

Dependence of the magnitude of the Stiles—Crawford effect on retinal location

1. The directional sensitivity (Stiles—Crawford effect) of retinal cones is supposed to be associated with their shape, but only extrafoveal cones have a cone-like shape; cones in the central fovea are elongated and look like rods.

2. To determine whether the directional sensitivity of cones depends on their shape, the Stiles—Crawford effect was measured both in the central fovea and in the parafovea of the human eye.

2. To ensure that the cone population tested was homogeneous, a small brief test flash, brought into the eye through the centre of the pupil, was placed at threshold by varying the intensity of a large adapting field. The directional sensitivity of the cones was determined by finding the efficiency of light to act as an adapting background as a function of position of entry in the pupil.

4. Central foveal cones have a less pronounced directional sensitivity than parafoveal cones and this lends support to the conclusion that the Stiles—Crawford effect is connected with the shape of the retinal receptors.

     

Phenomenology of the sound-induced flash illusion

Past studies, using pairings of auditory tones and visual flashes, which were static and coincident in space but variable in time, demonstrated errors in judging the temporal patterning of the visual flashes—the sound-induced flash illusion. These errors took one of the two forms: under-reporting (sound-induced fusion) or over-reporting (sound-induced fission) of the flash numbers. Our study had three objectives: to examine the robustness of both illusions and to consider the effects of stimulus set and response bias. To this end, we used an extended range of fixed spatial location flash–tone pairings, examined stimuli that were variable in space and time and measured confidence in judging flash numbers. Our results indicated that the sound-induced flash illusion is a robust percept, a finding underpinned by the confidence measures. Sound-induced fusion was found to be more robust than sound-induced fission and a most likely outcome when high numbers of flashes were incorporated within an incongruent flash–tone pairing. Conversely, sound-induced fission was the most likely outcome for the flash–tone pairing which contained two flashes. Fission was also shown to be strongly driven by stimuli confounds such as categorical boundary conditions (e.g. flash–tone pairings with ≤2 flashes) and compressed response options. These findings suggest whilst both fission and fusion are associated with ‘auditory driving’, the differences in the occurrence and strength of the two illusions not only reflect the separate neuronal mechanisms underlying audio and visual signal processing, but also the test conditions that have been used to investigate the sound-induced flash illusion.     

Menopausal hot flash and calciotonin gene-related peptide; effect of Keishi-bukuryo-gan,a kampo medicine,related to plasma calciotonin gene-related peptide level

OBJECTIVES: The purpose of this study is to investigate relationship of menopausal hot flash and calcitonin gene-related peptide (CGRP). Furthermore, this study evaluated the effect of the Japanese herbal (kampo) medicine Keishi-bukuryo-gan from the aspect of CGRP regulation. METHODS: Plasma CGRP and vasoactive intestinal peptide (VIP) levels were measured during hot flash and CGRP reactivity was studied by cold load test in subjects with/without hot flashes. The effect of Keishi-bukuryo-gan was assessed in comparison with plasma CGRP level. RESULTS: Only plasma CGRP but not VIP significantly elevated at the occurrence of hot flash (P=0.002). Stress by cold load significantly enhanced the over-secretion of CGRP in subjects with flash compared with those without flash (P=0.003) 3 min after the load. Keishi-bukuryo-gan decreased plasma CGRP level in subjects with hot flash. CONCLUSIONS: CGRP but not VIP was mainly related to the occurrence of hot flash. Keishi-bukuryo-gan, Japanese herbal medicine, improves hot flash possibly affecting plasma CGRP level.     

Surround contribution to light adaptation in cat retinal ganglion cells.

1. The sensitivity of a cat's retinal ganglion cell to a small, dim, spot flashed upon the middle of the receptive field depends upon the size of a concentric steady background: sensitivity is reduced monotonically with background area. All backgrounds which equal or exceed in size the central summing area of the ganglion cell produce an equivalent reduction of sensitivity, even though only backgrounds which extend outside the central summing area depress the maintained discharge. 2. If a small background lies upon the middle of the receptive field, and the test spot is made intense enough to evoke a strong response, steady illumination of the periphery may make the response larger. 3. This change in response is not due to an enhancement of centre sensitivity by the surround, but is readily understood if steady illumination of the periphery adapts out the surround's antagonism of the centre's response to the test flash. 4. The failure of steady stimulation of the surround to alter centre sensitivity implies that signals from the surround subtract from, or add to, those from the centre.     

How important is lateral masking in visual search?

Five experiments are presented, providing empirical support of the hypothesis that the sensory phenomenon of lateral masking may explain many well-known visual search phenomena that are commonly assumed to be governed by cognitive attentional mechanisms. Experiment I showed that when the same visual arrays are used in visual search and in lateral masking experiments, the factors (1) number of distractors, (2) distractor density, and (3) search type (conjunction vs disjunction) have the same effect on search times as they have on lateral masking scores. Experiment II showed that when the number of distractors and eccentricity is kept constant in a search task, the effect of reducing density (which reduces the lateral masking potential of distractors on the target) is to strongly reduce the disjunction-conjunction difference. In experiment III, the lateral masking potential of distractors on a target was measured with arrays that typically yield asymmetric search times in visual search studies (a Q among Os vs. an O among Qs). The lateral masking scores showed the same asymmetry. Experiment IV was a visual search study with such asymmetric search arrays in which the number of distractors and eccentricity was kept constant, while manipulating density. Reducing density (i.e., reducing lateral masking) produced a strong reduction of the asymmetry effect. Finally in experiment V, we showed that the data from experiment IV cannot be explained due to a difference between a fine and a coarse grain attentional mechanism. Taken together with eye movement data and error scores from experiment II and with similar findings from the literature, these results suggest that the sensory mechanism of lateral masking could well be a very important (if not the main) factor causing many of the well-known effects that are traditionally attributed to higher level cognitive or attentional mechanisms in visual search.