Andrew M. Haun

Perceived contrast in complex images

To understand how different spatial frequencies contribute to the overall perceived contrast of complex, broadband photographic images, we adapted the classification image paradigm. Using natural images as stimuli, we randomly varied relative contrast amplitude at different spatial frequencies and had human subjects determine which images had higher contrast. Then, we determined how the random variations corresponded with the human judgments. We found that the overall contrast of an image is disproportionately determined by how much contrast is between 1 and 6 c/°, around the peak of the contrast sensitivity function (CSF). We then employed the basic components of contrast psychophysics modeling to show that the CSF alone is not enough to account for our results and that an increase in gain control strength toward low spatial frequencies is necessary. One important consequence of this is that contrast constancy, the apparent independence of suprathreshold perceived contrast and spatial frequency, will not hold during viewing of natural images. We also found that images with darker low-luminance regions tended to be judged as having higher overall contrast, which we interpret as the consequence of darker local backgrounds resulting in higher band-limited contrast response in the visual system.

Is image quality a function of contrast perception

In this retrospective we trace in broad strokes the development of image quality measures based on the study of the early stages of the human visual system (HVS), where contrast encoding is fundamental. We find that while presenters at the Human Vision and Electronic Imaging meetings have frequently strived to find points of contact between the study of human contrast psychophysics and the development of computer vision and image quality algorithms. Progress has not always been made on these terms, although indirect impact of vision science on more recent image quality metrics can be observed.

Are we underestimating the richness of visual experience

Abstract It has been argued that the bandwidth of perceptual experience is low—that the richness of experience is illusory and that the amount of visual information observers can perceive and remember is extremely limited. However, the evidence suggests that this postulated poverty of experiential content is illusory and that visual phenomenology is immensely rich. To properly estimate perceptual content, experimentalists must move beyond the limitations of binary alternative-forced choice procedures and analyze reports of experience more broadly. This will open our eyes to the true richness of experience and to its neuronal substrates.

Using pattern classification to measure adaptation to the orientation of high order aberrations.

Background The image formed by the eyes optics is blurred by the ocular aberrations, specific to each eye. Recent studies demonstrated that the eye is adapted to the level of blur produced by the high order aberrations (HOA). We examined whether visual coding is also adapted to the orientation of the natural HOA of the eye. Methods and Findings Judgments of perceived blur were measured in 5 subjects in a psychophysical procedure inspired by the “Classification Images” technique. Subjects were presented 500 pairs of images, artificially blurred with HOA from 100 real eyes (i.e. different orientations), with total blur level adjusted to match the subjects natural blur. Subjects selected the image that appeared best focused in each random pair, in a 6-choice ranked response. Images were presented through Adaptive Optics correction of the subjects aberrations. The images selected as best focused were identified as positive, the other as negative responses. The highest classified positive responses correlated more with the subjects Point Spread Function, PSF, (r = 0.47 on average) than the negative (r = 0.34) and the difference was significant for all subjects (p<0.02). Using the orientation of the best fitting ellipse of angularly averaged integrated PSF intensities (weighted by the subjects responses) we found that in 4 subjects the positive PSF response was close to the subjects natural PSF orientation (within 21 degrees on average) whereas the negative PSF response was almost perpendicularly oriented to the natural PSF (at 76 degrees on average). Conclusions The Classification-Images inspired method is very powerful in identifying the internally coded blur of subjects. The consistent bias of the Positive PSFs towards the natural PSF in most subjects indicates that the internal code of blur appears rather specific to each subjects high order aberrations and reveals that the calibration mechanisms for normalizing blur also operate using orientation cues.

Binocular rivalry with peripheral prisms used for hemianopia rehabilitation.

To determine the relative binocular signal strength of moving images that are peripherally viewed through a monocular field expansion prism as opposed to moving images viewed directly. We hypothesised that prism blur might make prism images predominate less than images viewed directly with the other eye.

Adaptation to blurred and sharpened video

The visual system can distinguish different levels of blur and different levels of excess sharpness. Adaptation alters this capacity so that the adapted blur (or sharp) level looks more like a normal, properly focused image. Here, we describe the more general pattern of aftereffects of blur and sharp adaptation by measuring matching functions, using video clips from a DVD movie as stimuli. Results show that blur and sharp adaptation are selective: The sharpening aftereffects of blur adaptation are strongest for blurry videos while the blurring aftereffects of sharp adaptation are strongest for sharp videos. Despite the spatiotemporal variability of our adaptor and test stimuli, we found adaptation effects similar in magnitude to previous studies using invariant static images. A recent model of blur adaptation can be simplified to explain the form of our data, leading us to conclude that what we see as blur/sharp adaptation is a consequence of narrowband contrast adaptation.

Illusory bands in orientation and spatial frequency: a cortical analog to Mach bands.

Illusory bands at a luminance transition in space (ie an edge) are well known. Here we demonstrate illusory bands of enhanced orientations or spatial frequencies at transitions between higher-contrast and lower-contrast image content along the orientation and spatial-frequency dimensions—the dimensions of cortical spatial coding. We conclude that this illusion is a consequence of cortical-level suppression of units of similar orientations and spatial frequencies and serves to aid texture segmentation while providing efficient neural coding.

Contents of Consciousness Investigated as Integrated Information in Direct Human Brain Recordings

Integrated information theory postulates that the particular way stimuli appear when we consciously experience them arises from integrated information relationships across neural populations. We investigated if such equivalence holds by testing if similar/different percepts map onto similar/different information structures. We computed integrated information structure from intracranial EEGs recorded in 6 neurosurgical patients who had electrodes implanted over posterior cortices. During recording, we dissociated subjective percepts from physical inputs in three distinct stimulus paradigms (passive viewing, continuous flash suppression, and backward masking). Unsupervised classification showed that integrated information within stimulus-selective cortical regions classified visual experiences with significant accuracy (peaking on average around 64% classification accuracy). Classification by other relevant information theoretic measures such as mutual information and entropy was consistently poorer (56% and 54% accuracy). The findings argue that concepts from integrated information theory are empirically testable, promising a potential link between conscious experience and informational structures.

Similar Sensitivity to Ladder Contours in Macular Degeneration Patients and Controls.

Purpose To determine whether people with central field loss (CFL) from macular degeneration have improved ability to recognize a particularly difficult spatial configuration embedded in noise, the peripherally-viewed ‘ladder contour’. The visibility of these configuration has been linked to general contour integration ability and crowding limitations in peripheral vision. Methods We used a trial-based yes-no task. CFL patients and normally-sighted controls performed the task, looking for ladder contours embedded in a field of randomly oriented Gabor patches, at a range of stimulus presentation times (varying stimulus difficulty). Viewing eccentricity in CFL patients was set by their preferred retinal loci (PRLs) and matched artificially in the control group. The contours were presented so as to be tangent to the CFL region, given a patient’s PRL location. Results CFL and normally-sighted groups performed similarly on the task. The only significant determinant of performance was the viewing eccentricity. Conclusions CFL patients do not seem to develop any improved ability to recognize ladder contours with their parafoveal retina, which suggests that there is no underlying improvement in contour integration or reduction in crowding limitations in the region of the PRL despite extended daily use.

Plasticity in the Structure of Visual Space

Visual Abstract Visual space embodies all visual experiences, yet what determines the topographical structure of visual space remains unclear. Here we test a novel theoretical framework that proposes intrinsic lateral connections in the visual cortex as the mechanism underlying the structure of visual space. The framework suggests that the strength of lateral connections between neurons in the visual cortex shapes the experience of spatial relatedness between locations in the visual field. As such, an increase in lateral connection strength shall lead to an increase in perceived relatedness and a contraction in perceived distance. To test this framework through human psychophysics experiments, we used a Hebbian training protocol in which two-point stimuli were flashed in synchrony at separate locations in the visual field, to strengthen the lateral connections between two separate groups of neurons in the visual cortex. After training, participants experienced a contraction in perceived distance. Intriguingly, the perceptual contraction occurred not only between the two training locations that were linked directly by the changed connections, but also between the outward untrained locations that were linked indirectly through the changed connections. Moreover, the effect of training greatly decreased if the two training locations were too close together or too far apart and went beyond the extent of lateral connections. These findings suggest that a local change in the strength of lateral connections is sufficient to alter the topographical structure of visual space.