Recent studies suggest that the efficiency of cones to detect photons can be evaluated by measuring the equivalent input noise (EIN; derived from contrast thresholds measured in the presence and absence of visual noise) under specific conditions in which the contrast threshold is limited by the variability in the number of photons detected by photoreceptors (i.e., photon noise). These conditions can be identified based on the known properties of photon noise: spatially and temporally white and inversely proportional to the luminance intensity. The present study aims to adapt this psychophysical paradigm to evaluate the efficiency of rods to detect photons. A motion direction discrimination task was used to evaluate the EIN over a wide range of luminance intensities for various spatial and temporal frequencies when the display was blue or red (to which rods have little sensitivity). The target was either a Gabor patch presented at 20 degrees of eccentricity (first experiment) or a rotating sine-wave annulus with a radius of 10 degrees of eccentricity (second experiment). In both experiments, the EIN was found to be inversely proportional to luminance intensity over a limited range of luminance intensities for both display colors. At these luminance intensities, the EIN was roughly independent of the spatial and temporal frequencies, matching the properties of photon noise. Furthermore, under these conditions, contrast thresholds were lower (i.e., better) when the display was blue rather than red, which suggests that vision was mediated by rods when the display was blue. We conclude that the efficiency of rods to detect photons can be evaluated by measuring contrast thresholds in the presence and absence of visual noise over a limited range of luminance intensities with a blue display.

Tinnitus is the continuous phantom perception of a ringing in the ears. Recently, it has been suggested that tinnitus may be a maladaptive inference of the brain to auditory anomalies, whether they are detected or undetected by an audiogram. The present study presents empirical evidence for this hypothesis by inducing an illusion in a sensory domain that is damaged (auditory) and one that is intact (visual). It also presents novel information about how people with tinnitus process multisensory stimuli in the audio-visual domain.

Some motion information can be retained within iconic memory. Energy-based motion processing is known to be pre-attentive, whereas high-level motion processing can be described as attentively tracking the position of an object. The aim of the current study was to determine which motion information can be retained in iconic memory. Eight rotating sinewave gratings were presented simultaneously around an annulus of 5 degrees of visual angle. Each sinewave grating was presented in two 100-msec frames within which it was rotated 30 deg either clockwise or counterclockwise. The phase of the grating was either identical in the two frames or randomized, which provided local energy-based motion signals that were either strongly correlated or uncorrelated with the rotating direction thereby probing the energy-based or tracking motion system, respectively. Participants were asked to report the motion direction (i.e., clockwise vs counterclockwise) of the sinewave grating that was cued with an arrow presented at fixation. To modulate attentional resources dedicated to the target, the timing of the cue varied from 300 msec before the motion occurred (i.e., at the onset of the second frame) up to 400 msec after the motion occurred. When the phase of the sinewave grating was unchanged between the two frames, motion direction discrimination was good even when the cue was presented briefly after the motion occurred, which suggests that the energy-based rotational information was retained in iconic memory. When the phases of the two frames were uncorrelated, direction discrimination was possible only when the central cue was presented at least about 200 msec before the motion occurred and was at chance when the cue was presented briefly after the motion occurred, which suggests that high-level orientation tracking requires attention and cannot be retained in iconic memory.

Previous studies have shown that the number of photons detected by retinal cones can be estimated based on two contrast thresholds (with and without visual noise) measured under specific conditions. Indeed, contrast threshold can be limited by the variability in the number of photons detected (i.e., photon noise) under some conditions, in which case contrast threshold is inversely proportional to the square root of the luminance intensity. The present study aimed to adapt this psychophysical paradigm to estimate the number of photons detected by rods. Contrast thresholds to a motion direction discrimination task were measured in presence and absence of visual noise over a wide range of luminance intensities using either a red or blue monochromatic display. With a red display, contrast threshold in absence of noise was inversely proportional to the square root of the luminance intensity over a wide range of luminance intensities. At high luminance intensities, contrast thresholds were similar with the red and blue display at equal luminance intensities. At low luminance intensities, contrast thresholds were lower (i.e., better) with the blue display and inversely proportional to the square root of the luminance intensity. Given that rods are little sensitive to long wavelengths (i.e., red), contrast thresholds were undoubtedly limited by the photon noise of cones when using a red display and a blue display at high luminance intensities. The fact that contrast thresholds at low luminance intensities were better with the blue displays relative to the red display and since contrast threshold was inversely proportional to the square root of the luminance intensity under these conditions suggests performance was limited by the photon noise of rods. We conclude that the number of photons detected by rods can be evaluated by measuring motion contrast thresholds at low luminance intensities using a blue display.

Motion perception relies on two fundamentally different motion systems. The energy-based system relies on early direction-selective neurons that automatically and pre-attentively extract motion within their receptive fields. The high-level tracking system rather relies on attentively tracking the position of an object. Two types of apparent motion stimuli were used to investigate the two motion systems. Since early direction-selective neurons only operate within a short temporal window, a temporal gap of 100 msec between the two frames was used to probe the tracking system. No temporal gap was used to probe the energy-based system. In order to explore the role of attention on both systems, the attentional resources dedicated to the stimuli were systematically manipulated using a cueing paradigm. Eight dots simultaneously appeared uniformly distributed on an annulus with a radius of 5 degrees of visual angle. Each dot was displaced in a different random direction (up, down, right or left) in a 2-frame sequence. Observers were asked to report the displacement direction of the randomly selected dot (the target) selected by a central cue. The level of attentional resources dedicated to the target was manipulated by varying the timing of the central cue relative to the offset of the target. Results demonstrated that manipulating attention had a similar effect on both types of stimuli: percentage of correct answers was near perfect when the central cue appeared before the target onset and gradually declined when the cue appeared with or after the target). This suggests that attention plays a similar role in both systems. As energy-based processing is pre-attentive, these results propose that attention plays a minor role in tracking and that the drop in performance with a late cue might not be due to motion processing per se. Instead, it could be related to iconic memory.

Early direction-selective neurons in the primary visual cortex are widely considered to be the main neural basis underlying motion perception even though motion perception can also rely on attentively tracking the position of objects. Because of their small receptive fields, early direction-selective neurons suffer from the aperture problem, which is assumed to be overcome by integrating inputs from many early direction-selective neurons. Because the perceived motion of objects sometimes depends on static form information and does not always match the mean direction of local motion signals, the general consensus is that motion integration is form dependent and complex. Based on the fact that early direction-selective neurons respond to motion only within a short temporal window, the present study used stroboscopic motion to test their contribution to motion perception of objects. For conditions under which the perceived motion was impaired by stroboscopic motion, the perceived motion matched the mean direction of local motion signals and was form independent. For classic conditions under which the perceived motion could not be explained by a simple form independent averaging of local motion signals, neutralizing the contribution of early direction-selective neurons using stroboscopic motion had little impact on the perceived motion, which demonstrates that the perceived motion relied on position tracking, not on early direction-selective neurons. When the perceived motion relies on position tracking, assuming that motion perception relies on early direction-selective neurons can lead to erroneously postulate the existence of complex or form-dependent integration of inputs from early direction-selective neurons. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

The electroretinogram (ERG) is an objective measurement of the electrical response of retinal neurons, including photoreceptors, to light. The basic function of photoreceptors is to convert photons into neural signals. The present study investigated the capability of different ERG techniques to detect a reduced number of photons detected by photoreceptors compared to a functional assessment of the photon noise, which quantifies the number of photons detected by photoreceptors. The number of photons detected by photoreceptors were artificially reduced using a neutral density filter of 0.6, which reduced light intensity by a factor of 4. Three ERG techniques (full-field, pattern and multifocal) were performed for the measurement of photoreceptor electrical responses under a baseline and a reduced light intensity (i.e., neutral density filter) condition. The latency and amplitude of different retinogram waves were analyzed (full-field: a, b, flicker 30hz; pattern: P1 of each of the 5 rings; multifocal: P50, N95). The photon noise was derived from two contrast sensitivity measurements under specific conditions (presence and absence of noise, 0.5 cycles per degree, 2 Hz) using a motion direction discrimination task. The capability of each measurement (photon noise and various amplitudes and latencies of ERG techniques) to discriminate the baseline and reduced light intensity conditions was quantified using a ROC analysis. The results showed that no ERG parameter was more effective than the photon noise in discriminating between the baseline and reduced light intensity conditions (area under the ROC curve was 0.96 for the photon noise and ranged up to 0.84 for the various ERG parameters). We found no evidence that any ERG parameter would be more useful than the functional measurement of photon noise in detecting a reduced number of photons detected by photoreceptors.

The perceived motion direction of objects considerably relies on the motion signals along the edges of the object as demonstrated, for instance, in the barber-pole illusion. The barber-pole illusion is composed of drifting bars viewed through a rectangular aperture in which the short and long edges are oblique (45 deg) to the bars. In this illusion, motion tends to be perceived in the direction close to the long edges of the rectangular aperture instead of perpendicular to the bars (Fourier motion). The present study investigated how the motion signals along the edges in the barber-pole illusion are integrated into a global motion percept. Nine participants were asked to report the perceived motion direction when viewing the barber-pole illusion with their peripheral vision (25 degrees of eccentricity). The length of the rectangular aperture was systematically varied so that the long edges were 1, 2, 3 or 4 times longer than the short edges (1 creating a square-shaped aperture). The motion direction was perceived 1.4±0.8, 27.7±2.5, 35.2±2.1 and 38.4±1.9 degrees (mean±SE) from the Fourier motion (-45 and 45 degrees correspond to the motion direction along the short and long edges, respectively). The perceived motion direction could not be explained by a simple averaging of motion along the edges as this would be equivalent to attributing weights to edges that are proportional to their length. More specifically, this simple averaging would predict perceived motion directions of 0, 15, 22.5 and 27 degrees, respectively. On the other hand, attributing weights proportional to the square of the edge lengths would predict perceived motion directions of 0, 27, 36 and 39.7 degrees, respectively, which closely fits the data. We conclude that the visual system does not simply average the motion along the edges, it rather attributes considerably more weight to motion along the long edges.

Age-related decline in visual perception is usually attributed to optical factors of the eye and neural factors. However, the detection of light by cones converting light into neural signals is a crucial intermediate processing step of vision. Interestingly, a novel functional approach can evaluate many aspects of the visual system including the detection of photons by cones. This approach was used to investigate the underlying cause of age-related visual decline and found that the detection rate of cones was considerably affected with healthy aging. This functional test enabling to evaluate the detection of photons by cones could be particularly useful to screen for retinal pathologies affecting cones such as age-related macular degeneration. However, the paradigm used to functionally measure the detection of photons was complex as it was evaluating many other properties of the visual system. The aim of the current mini review is to clarify the underlying rationale of functionally evaluating the detection of photons by cones, describe a simpler approach to evaluate it, and review the impact of aging on the detection rate of cones.

Motor control deficits outlasting self-reported symptoms are often reported following mild traumatic brain injury (mTBI). The exact duration and nature of these deficits remains unknown. The current study aimed to compare postural responses to static or dynamic virtual visual inputs and during standard clinical tests of balance in 38 children between 9 and 18 years-of-age, at 2 weeks, 3 and 12 months post-concussion. Body sway amplitude (BSA) and postural instability (vRMS) were measured in a 3D virtual reality (VR) tunnel (i.e., optic flow) moving in the antero-posterior direction in different conditions. Measures derived from standard clinical balance evaluations (BOT-2, Timed tasks) and post-concussion symptoms (PCSS-R) were also assessed. Results were compared to those of 38 healthy non-injured children following a similar testing schedule and matched according to age, gender, and premorbid level of physical activity. Results highlighted greater postural response with BSA and vRMS measures at 3 months post-mTBI, but not at 12 months when compared to controls, whereas no differences were observed in post-concussion symptoms between mTBI and controls at 3 and 12 months. These deficits were specifically identified using measures of postural response in reaction to 3D dynamic visual inputs in the VR paradigm, while items from the BOT-2 and the 3 timed tasks did not reveal deficits at any of the test sessions. PCSS-R scores correlated between sessions and with the most challenging condition of the BOT-2 and as well as with the timed tasks, but not with BSA and vRMS. Scores obtained in the most challenging conditions of clinical balance tests also correlated weakly with BSA and vRMS measures in the dynamic conditions. These preliminary findings suggest that using 3D dynamic visual inputs such as optic flow in a controlled VR environment could help detect subtle postural impairments and inspire the development of clinical tools to guide rehabilitation and return to play recommendations.

In the Barber-Pole illusion (BPI), a drifting 1D texture is viewed through an elongated aperture and the perceived direction of motion is generally along the orientation of the aperture rather than the Fourier component (i.e., orthogonal to the 1D texture). According to the motion-path-integration theory, the BPI results from a greater path-motion energy along the aperture’s elongated orientation. Alternatively, the end-stop theory rather suggests that the perceived direction of motion is driven by the processing of line-ends along the contours of the aperture. Although there is good evidence supporting the end-stop theory for foveal viewing, its applicability for peripheral viewing (>10 degrees) has been questioned. The current study investigated the underlying cause of the BPI under peripheral viewing conditions (20 degrees) by systematically manipulating the shape of the aperture, which was an elongated parallelogram. When the long edges of the aperture were not parallel to the drifting texture, the perceived direction of motion was close to the orientation of the long edges (as typically perceived in the BPI) and the orientation of the short edges had little impact on the perceived direction of motion. However, when the long edges were parallel to the drifting texture (resulting in no motion along these edges), the perceived direction of motion was close to the orientation of the short edges. The fact that a small change in the contour orientation of the short edges had little impact on the global shape of the aperture, but drastically affected the perceived direction of motion, suggests that the BPI can be mainly driven by the processing of line-ends along the aperture contour even under peripheral viewing conditions. These results suggests that the BPI viewed peripherally does not depend on the global shape of the aperture per se, but rather on the contours of the aperture.

Motion perception is affected by healthy aging, which impairs the ability of older adults to perform some daily activities such as driving. The current study investigated the underlying causes of age-related motion contrast sensitivity losses by using an equivalent noise paradigm to decompose motion contrast sensitivity into calculation efficiency, the temporal modulation transfer function (i.e., temporal blur) and 3 sources of internal noise: stochastic absorption of photons by photoreceptors (i.e., photon noise), neural noise occurring at the retinal level (i.e., early noise) and at the cortical level (i.e., late noise). These sources of internal noise can be disentangled because there impacts on motion contrast sensitivity vary differently as a function of luminance intensity. The impact of healthy aging on these factors was evaluated by measuring motion contrast sensitivity of young and older healthy adults at different luminance intensities, temporal frequencies and with/without external noise. The older adults were found to have higher photon noise, which suggests a lower photon absorption rate of cones. When roughly equating the amount of photons being absorbed by the photoreceptors, older adults had lower calculation efficiencies, but no significant aging effect was found on temporal modulation transfer function, early noise and late noise.

Although early filters are narrowly tuned in the spatial frequency domain, distant spatial frequencies are not completely independent as cross-frequency interactions can facilitate (improve) or impair contrast sensitivity. For instance, a high-spatial frequency texture at high contrast can impair contrast sensitivity to low spatial frequencies, whereas at low contrast, it can facilitate contrast sensitivity. This surprising facilitation could be caused by late interactions within the visual system (e.g., integration of outputs from simple cells tuned to different spatial frequencies) or early interactions (e.g., cross-frequency lateral inhibition in the LGN). The current study investigated whether the cross-frequency interaction responsible of the facilitation effect occurs at a late stage where form and motion processing are distinct or at an earlier stage before form and motion processing have distinct pathways. If the cross-frequency interaction occurs at a stage at which form and motion processing are distinct, then static noise should have no impact on motion sensitivity. On the other hand, if the cross-frequency interaction occurs at a stage at which form and motion are not distinct, then static noise should have the same impact on contrast and motion sensitivity. We investigated the impact of static band-pass noise (2 to 8 cpd) on motion sensitivity of a 0.5-cpd Gabor drifting at temporal frequencies from 0.9375 to 15 Hz. The noise was full-screen and continuously displayed to avoid being spatial or temporally informative. The contrast of the noise was set to optimize the facilitation on contrast sensitivity based on a pilot study. Static noise was found to facilitate motion sensitivity at all temporal frequencies with a similar amplitude as observed for contrast sensitivity. The fact that static noise at high spatial frequencies can improve motion sensitivity suggests that the facilitation is caused by cross-frequency interactions occurring before form and motion are processed by distinct pathways.