Relation of spatial and temporal frequencies to vernier acuity

Vernier thresholds were measured with a pair of vertical sinusoidal gratings of one and a half cycles as targets. The amplitude was weighted by a one-dimensional Gaussian and contrast was set one log unit above contrast threshold. The vernier thresholds were estimated with the method of constant stimuli. Temporal frequency effects were introduced by movement of the vernier targets. It was found that vernier thresholds expressed in phase angle were unchanged in the effective range of spatial frequencies provided that the temporal frequency and the visibility were unchanged, and that thresholds deteriorated by increasing the temporal frequency. It is suggested that the detection of relative phase may be involved in the discrimination of vernier offsets and that it may be mediated by a sustained unit. Three possible types of mechanisms, edge-localization processes, orientation-selective units and phase-sensitive units, were considered in relation to vernier acuity.     

Cortical magnification within human primary visual cortex correlates with acuity thresholds

We measured linear cortical magnification factors in V1 with fMRI, and we measured visual acuity (Vernier and grating) in the same observers. The cortical representation of both Vernier and grating acuity thresholds in V1 was found to be roughly constant across all eccentricities. We also found a within-observer correlation between cortical magnification and Vernier acuity, further supporting claims that Vernier acuity is limited by cortical magnification in V1.     

Echo thresholds of the precedence effect in the vertical plane

Echo thresholds were measured for two configurations of loudspeakers in the vertical plane. The first configuration was characterized by the lead sound presentation from a loudspeaker placed in front of a subject, whereas the lag sound was presented from the loudspeaker above the subject's head. In the second configuration, the lead and lag sounds were presented from the same loudspeakers but in reverse order. All the stimuli were broadband noise bursts in the frequency range of 5-20 kHz. Burst durations were 5, 10, 20, and 100 ms. Average echo thresholds differences were significant only for the signals of 100 ms in duration (F (1, 16) = 6.28; p < 0.05). For the other signals (5, 10, 20 ms), there was no significant effect of location of lead and lag signals (p > 0.05).     

Form from motion parallax and form from luminance contrast: vernier discrimination

Some objects are perfectly camouflaged when stationary, but are clearly visible when moving; the boundaries of such an object are defined entirely by motion parallax. Little is known about the eye's ability to make spatial discriminations between motion-defined objects. In this study, subjects viewed a pseudo-random pattern of dots within which a camouflaged bar was made visible by relative motion of dots. Vernier acuity for the motion-defined bar was 27-45 sec arc for three subjects, much less than the interdot separation of 360 sec arc, much less than the 2 deg receptive field size for motion, and comparable with the foveal intercone separation of 30 sec arc. It is proposed that an opponent-orientation process and an opponent-position process can both contribute to vernier judgements for motion-defined objects. Real-world motion contrast commonly confounds the following cues for figure-ground segregation: (1) different texture velocities on either side of the figure's boundary; (2) in any given time interval, texture in figure and ground moves different distances; and (3) texture continually appears and disappears along the figure's boundary. When cues (2) and (3) were eliminated, thus ensuring figure-ground segregation was achieved entirely by motion-sensitive neural elements, vernier acuity was 44 +/- 5 sec arc compared with 36 +/- 8 sec arc for a dotted bar defined by luminance contrast. Conclusion: Vernier acuity for a dotted bar whose boundary was defined entirely by motion-sensitive neural elements was similar to vernier acuity for a dotted bar whose boundary was defined by luminance contrast.     

Visual acuity, contrast sensitivity and retinal magnification in a marsupial, the tammar wallaby (Macropus eugenii )

The visual acuity of the tammar wallaby was estimated using a behavioural discrimination task. The wallabies were trained to discriminate a high-contrast (86%) square-wave grating from a grey field of equal luminance (1000–6000 cd m⁻²). Visual-evoked cortical potentials were used to measure the complete contrast sensitivity function. The stimulus was a sinusoidal phase reversal of a sinusoidally modulated grating of various spatial frequencies and contrasts with a mean luminance of 40 cd m⁻². The behavioural acuity was estimated to be about 4.8 cycles/deg. The contrast sensitivity peaked at about 0.15 cycles/deg and declined towards both lower and higher spatial frequencies. The cut-off frequency of the contrast sensitivity function is slightly lower than the behaviourally measured acuity at about 2.7 cycles/deg. The retinal magnification factor was estimated anatomically from laser lesions to be about 0.16 mm/deg. Based on the known ganglion cell density and the retinal magnification factor, an anatomical upper limit to visual acuity of about 6 cycles/deg can be calculated. The differences in estimates of visual acuity between the behavioural and anatomical methods on the one side and physiology on the other side are discussed. Accepted: 28 May 1998     

Anisotropies in peripheral vernier acuity

Vernier acuity for short horizontal, vertical and oblique target lines was measured in many locations in the periphery of the visual field in normal human observers. In the 10 deg periphery, the average alignment threshold with oblique vernier lines in eight locations for three observers was 2.29 times higher than that with vertical and horizontal target lines. This oblique effect was found everywhere in the visual field. Similar conclusions are drawn for configurations in which the lines were replaced by just their distal endpoints, but here, additionally, performance was distinctly better when the dot pair was collinear with the fixation point, i.e. oriented radially, than when it was oriented tangentially. Both for vernier lines and for dot pairs, in all observers, horizontal configurations showed somewhat better thresholds than vertical ones. These results suggest an inherent pattern of connectivity throughout the visual field favoring processing in the cardinal orientations over the obliques, the radial over the tangential and, to a limited extent, the horizontal over the vertical.     

视觉诱发电位(VEP)方位效应与刺激位置的关系

以人视觉诱发电位(VEP)反应为指标,在视野的不同位置测定了VEP对四个方位的闪烁方波光栅刺激(时间频率2.9Hz,空间频率1.4c/deg,对比度0.94)的反应幅度。在距中央凹20°视角同心圆的八个刺激位置上,VEP反应幅度对与向心线垂直方位的光栅刺激(同心圆的切线方向),有统计意义上的优势。这一规律在垂直、水平向心线上尤为明显。从总体上未发现VEP反应幅度与刺激光栅方位有着明显的关系。这说明在人视野周边区,VEP反应幅度与光栅方位和向心线的夹角(偏向角)相关,而与光栅的绝对方位无关。在相同的刺激条件下,中央区的VEP反应幅度与刺激光栅方位之间也未发现明显关系。     

The detection of spatial discontinuities: interactions between contrast and spatial contiguity

Thresholds were measured for the detection of spatial discontinuities (notches and bumps) along luminance boundaries. At high contrasts of the boundary, thresholds expressed in terms of the spatial notch/bump height fell well inside the hyperacuity range. Expressed as luminance increment thresholds between adjacent photoreceptors, the same thresholds were similar to those previously reported by Hartridge and by Hecht and Mintz for the detection of a single line. The ability of observers to detect differences in the height of a boundary on either side of a mean luminance gap was also investigated, and the effect of the gap was found to depend upon stimulus contrast. At high contrasts the introduction of a gap increased thresholds, but at the lowest contrasts, thresholds were unaffected by a gap. The role of different spatial frequency and orientational mechanisms in vernier acuity is discussed.     

Gap detection in the starling (Sturnus vulgaris)

Summary Gap-detection thresholds of single units were determined from auditory forebrain neurons of the awake starling. Nine different response types were statistically defined from the discharge pattern to a 400 ms broadband noise stimulus. The gap stimuli consisted of two broadband noise bursts which were separated by a gap ranging from 0.4 to 204.8 ms duration. The median minimumdetectable gap for 121 out of 145 units that had a significant threshold 204.8ms was 12.8 ms; 20% of the neurons showed thresholds between 0.4 and 3.2 ms. The neurons of the nine response types differed significantly in their minimum-detectable gaps; neurons with phasic-tonic and phasic excitation exhibited the best (i.e. shortest) minimum-detectable gaps. The neurons of the three different recording areas (field L, NCM and HV) were significantly different in their minimumdetectable gaps; field L neurons showed the best temporal resolution for gaps in broadband noise. Gap-detection thresholds are compared with psychophysical thresholds determined with the same stimuli and the relevance of forebrain units for temporal resolution is discussed.Abbreviations CS control stimulus - HV hyperstriatum ventrale - HVc hyperstriatum ventrale pars caudalis - NB noise burst - NCM neostriatum caudale pars medialis - NS noise stimulus - SGS standard gap series - TW time window     

Orientation acuity estimated with simultaneous and successive procedures.

Orientation discrimination thresholds for medium contrast sinewave grating stimuli were estimated by a forced-choice procedure for the two principal, and the two main oblique axes. The results obtained for simultaneous presentation with a centre/surround stimulus arrangement are compared with the threshold estimates obtained when the two stimuli to be discriminated do not overlap in time. We find that orientation acuity depends on the temporal relationship of the discriminanda. Estimates of orientation threshold with simultaneous presentation are lower on all axes tested than those obtained with successive presentation; the greatest difference in orientation acuity was found at obliques. The data support the hypothesis that the meridional anisotropy in orientation discrimination that is found with conventional, two-interval forced-choice methods is not entirely attributable to anatomical and physiological changes at the level of the striate cortex.     

Mismatch negativity in the brain evoked potentials induced by short duration acoustic stimuli in adolescents in the norm and those with attention deficit

The effect of stimulus duration on the mismatch negativity of the auditory event-related potentials was studied in healthy children and those with attention-deficit symptoms. The minimum stimulus duration with which a MMN was elicited by frequency deviant was 50 ms in healthy children. The MMN was absent with shorter stimuli (11 ms and 30 ms). These parametres are close to those in adults. As to children with attention-deficit symptoms the MMN was insignificant with all tested stimulus durations (11 ms, 30 ms, 50 ms).     

Masking efficiency as a function of stimulus onset asynchrony for spatial-frequency detection and identification

Detection and identification thresholds for grating targets were measured in the presence of a compound mask grating as a function of the stimulus onset asynchrony (SOA). The detection and identification SOA functions are both reversed U-shaped but they are not parallel. The detection-to-identification ratio is itself a reversed U-shaped function of SOA, even for stimuli two octaves apart, with a peak between +20 and +60 ms SOA (backward masking). It is argued that these results support the hypothesis according to which detection and identification are serial processes.     

Orientation Transfer in Vernier and Stereoacuity Training

Human performance on various visual tasks can be improved substantially via training. However, the enhancements are frequently specific to relatively low-level stimulus dimensions. While such specificity has often been thought to be indicative of a low-level neural locus of learning, recent research suggests that these same effects can be accounted for by changes in higher-level areas–in particular in the way higher-level areas read out information from lower-level areas in the service of highly practiced decisions. Here we contrast the degree of orientation transfer seen after training on two different tasks—vernier acuity and stereoacuity. Importantly, while the decision rule that could improve vernier acuity (i.e. a discriminant in the image plane) would not be transferable across orientations, the simplest rule that could be learned to solve the stereoacuity task (i.e. a discriminant in the depth plane) would be insensitive to changes in orientation. Thus, given a read-out hypothesis, more substantial transfer would be expected as a result of stereoacuity than vernier acuity training. To test this prediction, participants were trained (7500 total trials) on either a stereoacuity (N = 9) or vernier acuity (N = 7) task with the stimuli in either a vertical or horizontal configuration (balanced across participants). Following training, transfer to the untrained orientation was assessed. As predicted, evidence for relatively orientation specific learning was observed in vernier trained participants, while no evidence of specificity was observed in stereo trained participants. These results build upon the emerging view that perceptual learning (even very specific learning effects) may reflect changes in inferences made by high-level areas, rather than necessarily fully reflecting changes in the receptive field properties of low-level areas.     

Local spatial scale for three-dot alignment acuity

Three-dot alignment discrimination thresholds were determined for blobs with Gaussian spatial and temporal contrast envelopes. The stimuli were presented at detection threshold luminance contrast. Thresholds were determined as a function of the blur parameter of the stimuli. This was done for a range of eccentricities in the visual field (from 45 degrees nasal to 65 degrees temporal). The thresholds were corrected for variations of the stimulus extent with the blur parameter. The results were used to estimate the local spatial scale for three-dot alignment acuity. This was done by a method recently introduced by Watson (1987). It was found that the local spatial scale for three-dot alignment acuity is approximately linearly proportional to eccentricity.     

Contrast and duration of exposure differentially affect vernier and stereoscopic acuity

Although stereoacuity and vernier acuity both yield comparable thresholds well below the eye's resolution limit, the neural circuits for these two classes of visual responses do not process the signals in an identical manner. It had previously been demonstrated that hyperacuity is more resistant to image blur than stereoacuity and that the zones within which two targets must be placed to achieve the lowest thresholds differ quite radically. Two further differences are reported here: reduced contrast affects stereoacuity more severely than hyperacuity, as also does shortening of exposure into a range of tens of milliseconds, even when the Bunsen-Roscoe-Bloch law has been factored out.     

Phase channels and the square-wave illusion

We have utilized a phase-sensitive effect, the square-wave illusion, to investigate the presence and tuning of phase-sensitive mechanisms. Subjects viewed a 1 c/deg triangular-wave grating and reported the presence or absence of the illusion, before and after adaptation to square-wave gratings of various spatial frequencies. Illusion strength declined with adaptation and was a function of spatial frequency. This function may reflect the spatial frequency tuning of a 'phase channel'. We have also measured by adaptation a contrast sensitivity function, maximally sensitive to a frequency of 1 c/deg. When normalized to the same maximal depression from baseline sensitivity, the phase channel and spatial-frequency channel shared the same low frequency tuning, but the phase channel was broader, with extended sensitivity to higher frequencies. These results can be understood if the phase channel is constructed from combinations of phase sensitive spatial-frequency channels.     

Temporal gap detection in tactile channels

The ability of observers to detect temporal gaps in bursts of sinusoids or bursts of band-limited noise was measured to assess the temporal acuity of Pacinian (P) and non-Pacinian (NP) tactile information processing channels. The P channel was isolated by delivering high frequency sinusoids or high frequency noise through a large 1.5-cm2 contactor to the thenar eminence. The NP channels were isolated from the P channel by delivering these stimuli as well as stimuli with lower frequencies through a small 0.01-cm2 contactor to the same site. Gap detection thresholds were higher for gaps in noise than for gaps in sinusoids but did not differ among conditions designed to isolate P and NP channels. The finding that temporal acuity does not differ among channels supports the hypothesis that, after termination of a stimulus, the P and NP channels exhibit the same amount of neural persistence. Also consistent with this hypothesis are the earlier findings that the enhancement of the sensation magnitude of a stimulus by a prior stimulus (Verrillo and Gescheider, Percept Psychophys 18: 128-136, 1975) and the duration of sensation after the termination of a stimulus (Gescheider et al., J Acoust Soc Am 91: 1690-1696, 1992) are independent of stimulus frequency. One important implication of this hypothesis, if true, is that the presence of temporal summation in the P channel and its absence in the NP channels, results, not from the lack of neural persistence in the NP channels, but instead, in marked contrast to the P channel, from the lack of a mechanism for integrating persistent neural activity over time.     

Further research on the functional asymmetry between the superior and inferior visual hemifields: analysis of the reaction time to simultaneous stimulation of the two hemi-fields

It has been previously reported that presentation of square-wave gratings to either side of the horizontal meridian of the visual field gives rise to different Simple Reaction Times (RTs), depending upon the spatial frequency of the stimuli. Specifically, for 1 c/deg stimulus RT is faster in the lower hemefield, whereas the reverse is true for 3 c/deg pattern, RT being faster in the upper visual field. In the reported experiment, RT to simultaneous presentation of either the same (alternatively 1 c/deg or 3 c/deg) or different (i.e. 1 c/deg and 3 c/deg combined) spatial frequencies to both hemifields was analyzed. The data show that whenever the two half components correspond to different RTs, the resulting RT equates that of the faster component. Conversely, when the two components give rise to identical RTs, the resulting RT does not differ from the value obtained with each half stimulus. Implications of this result for the functional organization of the visual system are discussed.     

White Noise Masks Some Temporal Parameters of the Auditory Localization of Radially Moving Targets

The temporal parameters of the perception of radially moving sound sources partly masked with broadband internalized noise at an intensity of 40, 46, or 52 dB above the hearing threshold have been studied. The threshold of sound duration necessary for identifying the direction of movement of the sound source (75% correct answers) increases from 135 ms in silence to 285 ms at all intensities of continuous noise studied. The minimum duration of the stimulus beginning with which a subsequent increase in duration does not increase the number of correct responses is the same (385 ms) under all conditions of stimulus presentation. Broadband noise of any intensity increases the time of response to stimuli in the range of durations studied. At a noise of 52 dB, which is close to the threshold of full masking, the reaction time is not increased significantly compared to its estimation at a noise of 46 dB. The minimum duration of the stimulus has proved to be the stablest temporal parameter of the perception of movement of a sound source. Changes in the temporal parameters of sound perception at noise levels close to the threshold of full masking are discussed.     

Behaviorally measured audiograms and gap detection thresholds in CBA/CaJ mice

Tone detection and temporal gap detection thresholds were determined in CBA/CaJ mice using a Go/No-go procedure and the psychophysical method of constant stimuli. In the first experiment, audiograms were constructed for five CBA/CaJ mice. Thresholds were obtained for eight pure tones ranging in frequency from 1 to 42 kHz. Audiograms showed peak sensitivity between 8 and 24 kHz, with higher thresholds at lower and higher frequencies. In the second experiment, thresholds for gap detection in broadband and narrowband noise bursts were measured at several sensation levels. For broadband noise, gap thresholds were between 1 and 2 ms, except at very low sensation levels, where thresholds increased significantly. Gap thresholds also increased significantly for low pass-filtered noise bursts with a cutoff frequency below 18 kHz. Our experiments revised absolute auditory thresholds in the CBA/CaJ mouse strain and demonstrated excellent gap detection ability in the mouse. These results add to the baseline behavioral data from normal-hearing mice which have become increasingly important for assessing auditory abilities in genetically altered mice.