Background. Feature and conjunction search tasks are widely used to study attentional deployment in space and in time. However, the spatiotemporal behavior of attention in these tasks remains under debate. Are multiple search stimuli processed in parallel or sequentially? Does this differ between these two types of search? If so, how? We used an innovative methodology to estimate the distribution of attention on a single trial basis for feature and conjunction search. Methods. Observers (n=8) performed feature and conjunction search tasks during separate sessions. They had to detect and discriminate a tilted low spatial-frequency grating among 3 similar looking distractors (i.e., vertical gratings in feature, and vertical or high spatial-frequency tilted gratings in conjunction search). After a variable delay, two additional probes (Landolt C's) were flashed at random locations. Performance in reporting the probes was used to infer attention deployment to those locations. Using the approach developed by Dugué et al. (PNAS 2015), we determined the probability of probe report at the most (P1) and least (P2) attended locations on a given trial. Were P1 and P2 equal, we would conclude that attention was uniformly distributed across the four locations occupied by the search stimuli. Otherwise, we would conclude that attention was non-uniformly distributed across the four locations. Results/Interpretations. Our results show that, for both feature and conjunction search, attention was non-uniformly distributed across the four locations. These results rule out a strict parallel/uniform model of attention processing during both the feature and conjunction search tasks. Interestingly, attentional distribution over time depended on the location of the probed stimuli in the visual field. This suggests that attentional distribution is heterogeneous across isoeccentric locations in a manner resembling asymmetries in visual performance at these locations. Meeting abstract presented at VSS 2016
ABSTRACT How do endogenous (voluntary) and exogenous (involuntary) attention modulate activity in visual cortex? Using ROI-based fMRI analysis, we measured fMRI activity for valid and invalid trials (target at cued/un-cued location, respectively), pre- or post-cueing endogenous or exogenous attention, while participants performed the same orientation discrimination task. We found stronger modulation in contralateral than ipsilateral visual regions, and higher activity in valid-than invalid-trials. For endogenous attention, modulation of stimulus-evoked activity due to a pre-cue increased along the visual hierarchy, but was constant due to a post-cue. For exogenous attention, modulation of stimulus-evoked activity due to a pre-cue was constant along the visual hierarchy, but was not modulated due to a post-cue. These findings reveal that endogenous and exogenous attention distinctly modulate activity in visuo-occipital areas during orienting and reorienting; endogenous attention facilitates both the encoding and the readout of visual information whereas exogenous attention only facilitates the encoding of information.
Introduction: Attention Deficit Hyperactivity Disorder (ADHD), a behavioral disorder characterized by inappropriate levels of inattention, hyperactivity and impulsivity, consists of a diverse combination of cognitive deficits, and not all individuals diagnosed with ADHD present all deficits. For instance, many people with ADHD experience difficulties in sustaining attention over time but this impairment is not evident in every individual. Recent studies suggest that ADHD is associated with an impaired ability to anticipate predicable events. Our hypothesis is that the impairment of temporal expectation is related to deficits in sustained attention and therefore the two impairments are expected to co-occur in the same individuals. We utilized a novel approach for studying temporal expectation by examining the inhibition of microsaccades (saccades < 1deg), prior to the onset of predicted stimuli. Methods: 20 participants diagnosed with ADHD and 20 neurotypical participants performed two Continuous Performance Tasks (CPTs) with fixed and random inter-stimulus intervals, while their eye movements were recorded. We estimated "predictive microsaccade inhibition" (PMSI) by comparing microsaccade-rate prior to predictable stimuli (presented in fixed intervals) with microsaccade-rate prior to unpredictable stimuli (presented in random intervals). We divided each group of observers according to their sustained attention performance into two subgroups: 'high' performers and 'low' performers. Results: We found that: (a) Individuals with ADHD showed a smaller PMSI than controls. (b) Within each group, the PMSI was significantly stronger for high- than low-performers. But, when dividing each group according to the Adult ADHD Self-Report scale (ASRS), no such difference was observed. Conclusions: The results show that sustained attention is tightly linked to temporal expectations, as measured by PMSI. The attenuated PMSI in ADHD may impair performance. These results demonstrate that PMSI can be a powerful tool for studying temporal expectations in clinical and neurotypical populations. Meeting abstract presented at VSS 2016
Damage to the primary visual cortex typically causes cortical blindness (CB) in the hemifield contralateral to the damaged hemisphere. Recent evidence indicates that visual training can partially reverse CB at trained locations. Whereas training induces near-complete recovery of coarse direction and orientation discriminations, deficits in fine motion processing remain. Here, we systematically disentangle components of the perceptual inefficiencies present in CB fields before and after coarse direction discrimination training. In seven human CB subjects, we measured threshold versus noise functions before and after coarse direction discrimination training in the blind field and at corresponding intact field locations. Threshold versus noise functions were analyzed within the framework of the linear amplifier model and the perceptual template model. Linear amplifier model analysis identified internal noise as a key factor differentiating motion processing across the tested areas, with visual training reducing internal noise in the blind field. Differences in internal noise also explained residual perceptual deficits at retrained locations. These findings were confirmed with perceptual template model analysis, which further revealed that the major residual deficits between retrained and intact field locations could be explained by differences in internal additive noise. There were no significant differences in multiplicative noise or the ability to process external noise. Together, these results highlight the critical role of altered internal noise processing in mediating training-induced visual recovery in CB fields, and may explain residual perceptual deficits relative to intact regions of the visual field.
Background and Goal. Covert spatial attention modulates behavioral and neural sensitivity. fMRI studies have reported that endogenous (voluntary) attention increases BOLD amplitude, shifts population receptive fields (pRFs) and alters pRF sizes. Here, we used a combined fMRI/psychophysics experiment to investigate these effects concurrently, at polar angle locations that typically show discriminability differences. Methods. In every trial, a precue directed participants to either attend to one of four isoeccentric (6°) locations on the cardinal meridians, or to distribute attention across four locations. 300 ms after the precue, a stimulus for mapping pRFs (a contrast pattern masked by a bar aperture) was presented for 1 or 2 s. Shortly after, four small, low-contrast Gabor patches appeared and participants discriminated the orientation of the target Gabor indicated by a response cue. PRF models were solved for voxels in V1-hV4 and V3A/B. Results. Focal attention improved behavioral performance at the cued location and decreased performance at the other locations to the same extent across the four locations. In all visual field maps, BOLD amplitude increased for voxels with pRF centers near the attended location, and decreased at unattended locations. The amplitude changes were independent of mapping stimulus location, reflecting a baseline shift rather than a multiplicative gain. The magnitude and spatial spread of amplitude changes were similar across locations and maps. pRF centers shifted slightly towards the cued location and there was a trend for smaller peripheral pRF sizes in the focal than the distributed attention condition. These two effects increased across the visual hierarchy. Conclusions. We observed a pronounced attention-related baseline shift in BOLD response, accompanied by small but detectable changes in properties of visual field maps. Our results suggest that endogenous spatial attention, prior to target appearance, primarily affects visual cortex by a retinotopic change in mean neural activity.
Goal: Voluntary temporal attention, the prioritization of visual information at task-relevant points in time, improves perceptual sensitivity. However, little is known about how temporal attention affects the neural representation of visual information. Here we investigated whether and how temporal attention affects orientation representations. Methods: In two experiments, we used a two-target temporal cueing task to manipulate voluntary temporal attention and measured the effects on orientation representations using MEG decoding. On each trial, two grating targets (T1 and T2), each independently tilted around vertical or horizontal, appeared sequentially at the same location, separated by a 300-ms interval. An auditory precue (75% validity) before the targets instructed observers to attend to T1 or T2. An auditory response cue after the targets instructed observers to report the tilt (clockwise or counter-clockwise) of either T1 or T2. In trials where the precue directed temporal attention to T1, T1 was attended and T2 was unattended, and vice versa when the precue was to T2. The temporal cueing protocol was similar in the two experiments, except that in Experiment 1, the targets were presented on a gray background, whereas in Experiment 2, the targets were superimposed on 20-Hz flickering noise patches. To measure time-resolved orientation representations for each target, we decoded vertical vs. horizontal orientation across time using a linear support vector machine. Results: In both experiments, temporal attention improved behavioral accuracy, and it increased orientation decoding accuracy for T1. The improvement occurred 235-300 ms and 195-260 ms after T1 onset for Experiments 1 and 2, respectively, according to cluster permutation tests. There was no evidence for improved T2 decoding accuracy in either experiment. Conclusions: Voluntary temporal attention enhanced the orientation representation of the first target. This sensory-level change reveals a possible neural mechanism of how temporal attention could improve visual perceptual sensitivity.
After baseline measurements in-lab, participants were sent home to train for several months (subacute (SA): 4.9±0.6 months [mean ± SD]; chronic (CH): 6.8±4.0 months). They used their personal computers with a chin/forehead-rest provided by the lab, which they were instructed to position 42cm away from their display during training. Participants performed 300 trials of their assigned training tasks (Static, Motion or Flicker) per location per day, at least five days per week, and they emailed their data log files back to the lab for analysis every week. During home training sessions, they were instructed to stay fixated on the fixation spot and warned that inadequate fixation accuracy could limit recovery.Session thresholds were calculated by fitting a Weibull function with a 72.5 percent correct performance threshold criterion. After participants’ thresholds improved consistently for at least 20 sessions, we moved their training stimulus 1˚ deeper into the blind field along the x-axis (Cartesian coordinate space). Because the diameter of the stimulus is 2.5 ˚, the new training location had ~80% overlap with the original training location.Once participants trained for about 4 months, with at least one improved location (defined as consistently good contrast thresholds at that location), they were brought back to the lab, and performance at all home-trained locations was verified with eye tracker-enforced fixation control. We aimed for a similar number of training sessions at the blind-field locations of interest before scheduling people to return for in-lab performance verification. However, the amount of time elapsed until the return visit did vary, as it was affected by the individual’s rate of improvement, their work/family schedules, and ability to travel to our single study site (participants originated from across the entire United States and Canada). All participants were best corrected using glasses or contact lenses during testing and training. The Research Subjects Review Board approved study procedures at the University of Rochester, which were conducted as per the Declaration of Helsinki, with written informed consent obtained from each participant, and participation was voluntary.
Abstract Human visual performance for basic visual dimensions (e.g., contrast sensitivity and acuity) peaks at the fovea and decreases with eccentricity. The eccentricity effect is related to the larger surface area of the visual cortex corresponding to the fovea, but it is unknown if differential feature tuning contributes to this eccentricity effect. Here, we investigated two system-level computations underlying the eccentricity effect: featural representation (tuning) and internal noise. Observers (both sexes) detected a Gabor embedded in filtered white noise which appeared at the fovea or one of four perifoveal locations. We used psychophysical reverse correlation to estimate the weights assigned by the visual system to a range of orientations and spatial frequencies (SFs) in noisy stimuli, which are conventionally interpreted as perceptual sensitivity to the corresponding features. We found higher sensitivity to task-relevant orientations and SFs at the fovea than the perifovea, and no difference in selectivity for either orientation or SF. Concurrently, we measured response consistency using a double-pass method, which allowed us to infer the level of internal noise by implementing a noisy observer model. We found lower internal noise at the fovea than perifovea. Finally, individual variability in contrast sensitivity correlated with sensitivity to and selectivity for task-relevant features as well as with internal noise. Moreover, the behavioral eccentricity effect mainly reflects the foveal advantage in orientation sensitivity compared to other computations. These findings suggest that the eccentricity effect stems from a better representation of task-relevant features and lower internal noise at the fovea than at the perifovea. Significance Performance in many visual tasks worsens with eccentricity. Many studies attribute this eccentricity effect to retinal and cortical factors, like higher cone density and a larger cortical surface area representing the foveal than peripheral locations. We investigated whether system-level computations for task-relevant visual features also underlie this eccentricity effect. Measuring contrast sensitivity in visual noise, we showed that the fovea better represents task-relevant orientation and spatial frequency and has lower internal noise than the perifovea, and that individual variability in these two computations correlates with that in performance. These findings reveal that both representations of these basic visual features and internal noise underlie the difference in performance with eccentricity.
Cortical blindness (CB), vision loss resulting from damage to the primary visual cortex or its immediate afferents, can be partially alleviated through visual perceptual training1. However, learning does not transfer deeper within the blind field2,3. Given that exogenous spatial attention (ESA) enables perceptual learning4 and facilitates transfer5, here, we sought to reduce the spatial specificity of training by manipulating ESA. Chronic CB subjects trained on left-right, global direction integration just inside the blind field. In Flash training (n=3), a pre-cue (5-deg disc, 15Hz flicker, 200ms duration) appeared 7-deg deeper in the blind field. In Exposure training (n=3), an identical stimulus appeared simultaneously, 7-deg deeper in the blind field. Pre-cues (1-deg discs, 15Hz flicker, 200ms duration) appeared above both stimuli. For all conditions, blind-field pre-training performance approximated chance. Both ESA training conditions improved performance at trained locations, but Normalized Direction Range (NDR) thresholds following Flash training (57±16%) were poorer than after both Exposure training (35±5%), and single stimulus training without pre-cues (27±9%); the last two groups did not differ (p>.1). None of the three groups exhibited learning at the deeper locations. Thus, the Flash pre-cue reduced training efficacy in the blind field, possibly by withdrawing attentional resources from the training location into a location where no stimulus was presented. Our findings show that directing ESA deep in the blind field attracts attention automatically and hinders performance if no relevant task information is presented at that location, revealing that exogenous attention exerts its effects even in the blind field.
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