Steven C. Dakin

Adaptable history biases in human perceptual decisions

Significance Adapting to the environment requires using feedback about previous decisions to make better future decisions. Sometimes, however, the past is not informative and taking it into consideration leads to worse decisions. In psychophysical experiments, for instance, humans use past feedback when they should ignore it and thus make worse decisions. Those choice history biases persist even in disadvantageous contexts. To test this persistence, we adjusted trial sequence statistics. Subjects adapted strongly when the statistics confirmed their biases, but much less in the opposite direction; existing biases could not be eradicated. Thus, even in our simplest sensory decisions, we exhibit a form of confirmation bias in which existing choice history strategies are easier to reinforce than to relinquish. When making choices under conditions of perceptual uncertainty, past experience can play a vital role. However, it can also lead to biases that worsen decisions. Consistent with previous observations, we found that human choices are influenced by the success or failure of past choices even in a standard two-alternative detection task, where choice history is irrelevant. The typical bias was one that made the subject switch choices after a failure. These choice history biases led to poorer performance and were similar for observers in different countries. They were well captured by a simple logistic regression model that had been previously applied to describe psychophysical performance in mice. Such irrational biases seem at odds with the principles of reinforcement learning, which would predict exquisite adaptability to choice history. We therefore asked whether subjects could adapt their irrational biases following changes in trial order statistics. Adaptability was strong in the direction that confirmed a subject’s default biases, but weaker in the opposite direction, so that existing biases could not be eradicated. We conclude that humans can adapt choice history biases, but cannot easily overcome existing biases even if irrational in the current context: adaptation is more sensitive to confirmatory than contradictory statistics.

Spatial-frequency dependent binocular imbalance in amblyopia

While amblyopia involves both binocular imbalance and deficits in processing high spatial frequency information, little is known about the spatial-frequency dependence of binocular imbalance. Here we examined binocular imbalance as a function of spatial frequency in amblyopia using a novel computer-based method. Binocular imbalance at four spatial frequencies was measured with a novel dichoptic letter chart in individuals with amblyopia, or normal vision. Our dichoptic letter chart was composed of band-pass filtered letters arranged in a layout similar to the ETDRS acuity chart. A different chart was presented to each eye of the observer via stereo-shutter glasses. The relative contrast of the corresponding letter in each eye was adjusted by a computer staircase to determine a binocular Balance Point at which the observer reports the letter presented to either eye with equal probability. Amblyopes showed pronounced binocular imbalance across all spatial frequencies, with greater imbalance at high compared to low spatial frequencies (an average increase of 19%, pu2009<u20090.01). Good test-retest reliability of the method was demonstrated by the Bland-Altman plot. Our findings suggest that spatial-frequency dependent binocular imbalance may be useful for diagnosing amblyopia and as an outcome measure for recovery of binocular vision following therapy.

Local and Global Limits on Visual Processing in Schizophrenia

Schizophrenia has been linked to impaired performance on a range of visual processing tasks (e.g. detection of coherent motion and contour detection). It has been proposed that this is due to a general inability to integrate visual information at a global level. To test this theory, we assessed the performance of people with schizophrenia on a battery of tasks designed to probe voluntary averaging in different visual domains. Twenty-three outpatients with schizophrenia (mean age: 40±8 years; 3 female) and 20 age-matched control participants (mean age 39±9 years; 3 female) performed a motion coherence task and three equivalent noise (averaging) tasks, the latter allowing independent quantification of local and global limits on visual processing of motion, orientation and size. All performance measures were indistinguishable between the two groups (ps>0.05, one-way ANCOVAs), with one exception: participants with schizophrenia pooled fewer estimates of local orientation than controls when estimating average orientation (p = 0.01, one-way ANCOVA). These data do not support the notion of a generalised visual integration deficit in schizophrenia. Instead, they suggest that distinct visual dimensions are differentially affected in schizophrenia, with a specific impairment in the integration of visual orientation information.

Metamorphopsia and letter recognition.

Acuity is the most commonly used measure of visual function, and reductions in acuity are associated with most eye diseases. Metamorphopsia--a perceived distortion of visual space--is another common symptom of visual impairment and is currently assessed qualitatively using Amsler (1953) charts. In order to quantify the impact of metamorphopsia on acuity, we measured the effect of physical spatial distortion on letter recognition. Following earlier work showing that letter recognition is tuned to specific spatial frequency (SF) channels, we hypothesized that the effect of distortion might depend on the spatial scale of visual distortion just as it depends on the spatial scale of masking noise. Six normally sighted observers completed a 26 alternate forced choice (AFC) Sloan letter identification task at five different viewing distances, and the letters underwent different levels of spatial distortion. Distortion was controlled using spatially band-pass filtered noise that spatially remapped pixel locations. Noise was varied over five spatial frequencies and five magnitudes. Performance was modeled with logistic regression and worsened linearly with increasing distortion magnitude and decreasing letter size. We found that retinal SF affects distortion at midrange frequencies and can be explained with the tuning of a basic contrast sensitivity function, while object-centered distortion SF follows a similar pattern of letter object recognition sensitivity and is tuned to approximately three cycles per letter (CPL). The interaction between letter size and distortion makes acuity an unreliable outcome for metamorphopsia assessment.

Visual Population Receptive Fields in People with Schizophrenia Have Reduced Inhibitory Surrounds

People with schizophrenia (SZ) experience abnormal visual perception on a range of visual tasks, which have been linked to abnormal synaptic transmission and an imbalance between cortical excitation and inhibition. However, differences in the underlying architecture of visual cortex neurons, which might explain these visual anomalies, have yet to be reported in vivo. Here, we probed the neural basis of these deficits using fMRI and population receptive field (pRF) mapping to infer properties of visually responsive neurons in people with SZ. We employed a difference-of-Gaussian model to capture the center-surround configuration of the pRF, providing critical information about the spatial scale of the pRFs inhibitory surround. Our analysis reveals that SZ is associated with reduced pRF size in early retinotopic visual cortex, as well as a reduction in size and depth of the inhibitory surround in V1, V2, and V4. We consider how reduced inhibition might explain the diverse range of visual deficits reported in SZ. SIGNIFICANCE STATEMENT People with schizophrenia (SZ) experience abnormal perception on a range of visual tasks, which has been linked to abnormal synaptic transmission and an imbalance between cortical excitation/inhibition. However, associated differences in the functional architecture of visual cortex neurons have yet to be reported in vivo. We used fMRI and population receptive field (pRF) mapping to demonstrate that the fine-grained functional architecture of visual cortex in people with SZ differs from unaffected controls. SZ is associated with reduced pRF size in early retinotopic visual cortex largely due to reduced inhibitory surrounds. An imbalance between cortical excitation and inhibition could drive such a change in the center-surround pRF configuration and ultimately explain the range of visual deficits experienced in SZ.

Binocular Therapy for Childhood Amblyopia Improves Vision Without Breaking Interocular Suppression

PurposenAmblyopia is a common developmental visual impairment characterized by a substantial difference in acuity between the two eyes. Current monocular treatments, which promote use of the affected eye by occluding or blurring the fellow eye, improve acuity, but are hindered by poor compliance. Recently developed binocular treatments can produce rapid gains in visual function, thought to be as a result of reduced interocular suppression. We set out to develop an effective home-based binocular treatment system for amblyopia that would engage high levels of compliance but that would also allow us to assess the role of suppression in childrens response to binocular treatment.nnnMethodsnBalanced binocular viewing therapy (BBV) involves daily viewing of dichoptic movies (with visibility matched across the two eyes) and gameplay (to monitor compliance and suppression). Twenty-two children (3-11 years) with anisometropic (n = 7; group 1) and strabismic or combined mechanism amblyopia (group 2; n = 6 and 9, respectively) completed the study. Groups 1 and 2 were treated for a maximum of 8 or 24 weeks, respectively.nnnResultsnThe treatment elicited high levels of compliance (on average, 89.4% ± 24.2% of daily dose in 68.23% ± 12.2% of days on treatment) and led to a mean improvement in acuity of 0.27 logMAR (SD 0.22) for the amblyopic eye. Importantly, acuity gains were not correlated with a reduction in suppression.nnnConclusionsnBBV is a binocular treatment for amblyopia that can be self-administered at home (with remote monitoring), producing rapid and substantial benefits that cannot be solely mediated by a reduction in interocular suppression.

Visual acuity loss in patients with age-related macular degeneration measured using a novel high-pass letter chart

Background/aims Conventional Logarithm of the Minimum Angle of Resolution (logMAR) acuity is the current gold standard for assessing visual function in age-related macular degeneration (AMD). However, visual acuity (VA) often remains ‘normal’ when measured with these charts, even with advanced retinal changes. We wished to investigate how VA measurements with the Moorfields Acuity Chart (MAC), which employs high-pass filtered letters, compares to conventional letter charts in subjects with AMD. Methods Monocular best-corrected VA measurements and test–retest variability (TRV) were compared for conventional and MAC charts in 38 normal observers (mean age 52.1u2005years) and 80 patients (mean age 80.6u2005years) with varying degrees of acuity loss owing to AMD. Methods of Bland–Altman and ordinary least-squares regression were employed for data analysis. Results A proportional bias was confirmed between conventional and MAC measurements (r2=0.133, p=0.001) such that MAC acuity was −0.45 logMAR ‘worse’ at the 0.00 logMAR acuity level, but only −0.26 logMAR ‘worse’ at the 1.00 logMAR level. The mean bias was much smaller in the normal subject group (−0.16 logMAR). Similar TRV (ranging from ±0.09 to ±0.12 logMAR) was found for both charts in both subject groups. Conclusions VA measurements with the MAC chart appear to be more sensitive to functional loss in AMD compared with conventional letter charts, with similar TRV. Simulations indicate this may be because the high-pass filtered letters are more vulnerable to undersampling as a result of retinal cell loss in the disease process.

Metamorphopsia and interocular suppression in monocular and binocular maculopathy.

–1.0 D OD and –0.25 D OS. Her IOP was 14 mmHg OD and 15 mmHg OS. Ophthalmoscopy revealed bilateral neuroretinal rim narrowing at the optic disc margin, inferior retinal nerve fibre layer (RNFL) defect and peripapillary retinoschisis without a posterior vitreous detachment in the right eye with normal open anterior chamber angle (Fig. 1). Fluorescein angiography showed no leakage or any signs of an optic disc pit. Optical coherence tomography (OCT, OCT3-STRATUS, Carl Zeiss Meditec) revealed peripapillary retinoschisis and retinal detachment. Humphrey field analyzer (HumphreyZeiss System) showed a superior Bjerrum scotoma corresponding to the inferior RNFL defect in both eyes. She was followed without any treatment because of her good vision. The headaches and periorbital pain disappeared after 1 month. She noticed that the paracentral scotoma had moved gradually towards the central visual field, and the peripapillary retinoschisis turned to macular retinoschisis at 15 months but she maintained good vision of 20/20. The macular retinoschisis completely resolved after 3 years without any recurrence for 2 years. Vision was maintained at 20/20 without a central scotoma. The IOP remained within the normal range without any topical medication and no progression of the visual field defect was detected. Spectral domain OCT (Cirrus-HD-OCT, Carl Zeiss Meditec) showed a resolution of the macular retinoschisis with partially detachment of the posterior vitreous cortex. The optic disc cup was deeper corresponding to the resolution of the peripapillary retinoschisis and schisis within the optic disc. Kahook et al. (2007) described two cases of peripapillary retinoschisis associated with increased IOP and angle-closure glaucoma. An acute elevation of the IOP can lead to structural defects in the optic nerve head and peripapillary retinoschisis as seen in cases of optic disc pit maculopathy. Zumbro et al. (2007) reported that vitreous surgery can resolve a retinoschisis associated with an enlarged optic disc cup. They reported that vitreous traction may have played a role in the development of the macular retinoschisis and foveal detachment. Zhao & Li (2011) described a case of macular retinoschisis that developed in an eye with normal tension glaucoma. They also suggested that vitreous traction was the cause of the retinoschisis near the RNFL defect, and the foveal detachment developed by the seeping of fluid through the intraretinal spaces. The IOP was normal as in our patient, and vitreous traction probably caused the peripapillary and macular retinoschisis that resolved spontaneously when the posterior vitreous cortex was partially detached. Hwang et al. (2014) described a case of peripapillary retinoschisis within the retinal nerve fibre, ganglion cell and inner plexiform layers detected in the OCT images of 19 glaucomatous eyes. However, none of the eyes developed macular retinoschisis, and the peripapillary retinoschisis resolved without any treatment. Our patient developed symptomatic peripapillary retinoschisis and retinal detachment which expanded to macular retinoschisis. Thus, peripapillary retinoschisis associated with glaucomatous optic neuropathy can progress to macular retinoschisis which can resolve spontaneously by a partial vitreous detachment.

Prior object-knowledge sharpens properties of early visual feature-detectors

Early stages of visual processing are carried out by neural circuits activated by simple and specific features, such as the orientation of an edge. A fundamental question in human vision is how the brain organises such intrinsically local information into meaningful properties of objects. Classic models of visual processing emphasise a one-directional flow of information from early feature-detectors to higher-level information-processing. By contrast to this view, and in line with predictive-coding models of perception, here, we provide evidence from human vision that high-level object representations dynamically interact with the earliest stages of cortical visual processing. In two experiments, we used ambiguous stimuli that, depending on the observer’s prior object-knowledge, can be perceived as either coherent objects or as a collection of meaningless patches. By manipulating object knowledge we were able to determine its impact on processing of low-level features while keeping sensory stimulation identical. Both studies demonstrate that perception of local features is facilitated in a manner consistent with an observer’s high-level object representation (i.e., with no effect on object-inconsistent features). Our results cannot be ascribed to attentional influences. Rather, they suggest that high-level object representations interact with and sharpen early feature-detectors, optimising their performance for the current perceptual context.

The Auckland Optotypes: An open-access pictogram set for measuring recognition acuity.

When measuring recognition acuity in a research setting, the most widely used symbols are the Early Treatment of Diabetic Retinopathy Study (ETDRS) set of 10 Sloan letters. However, the symbols are not appropriate for patients unfamiliar with letters, and acuity for individual letters is variable. Alternative pictogram sets are available, but are generally comprised of fewer items. We set out to develop an open-access set of 10 pictograms that would elicit more consistent estimates of acuity across items than the ETDRS letters from visually normal adults. We measured monocular acuity for individual uncrowded optotypes within a newly designed set (The Auckland Optotype [TAO]), the ETDRS set, and Landolt Cs. Eleven visually normal adults were assessed on regular and vanishing formats of each set. Inter-optotype reliability and ability to detect subtle differences between participants were assessed using intraclass correlations (ICC) and fractional rank precision (FRP). The TAO vanishing set showed the strongest performance (ICC = 0.97, FRP = 0.90), followed by the other vanishing sets (Sloan ICC = 0.88, FRP = 0.74; Landolt ICC = 0.86, FRP = 0.80). Within the regular format, TAO again outperformed the existing sets (TAO ICC = 0.77, FRP = 0.75; Sloan ICC = 0.65, FRP = 0.64; Landolt ICC = 0.48, FRP = 0.63). For adults with normal visual acuity, the new optotypes (in both regular and vanishing formats) are more equally legible and sensitive to subtle individual differences than their Sloan counterparts. As this set does not require observers to be able to name Roman letters, and is freely available to use and modify, it may have wide application for measurement of acuity.