Performance in visual tasks improves with attention, and this improvement has been shown to stem, in part, from changes in sensory processing. However, the mechanism by which attention affects perception remains unclear. Considering that neurons within the visual areas are selective for basic image statistics, such as orientation or spatial frequency (SF), it is plausible that attention modulates these sensory preferences by altering their so-called ‘tuning curves’. The goal of this project is to investigate this possibility by measuring and comparing the SF tuning curves across a range of attentional states in humans. In Experiment 1, a model-driven approach to fMRI analysis was introduced that allows for fast and efficient estimation of population spatial frequency tuning (pSFT) for individual voxels within human visual cortices. Using this method, I estimated pSFTs within early visual cortices of 8 healthy, young adults. Consistent with previous studies, the estimated SF optima showed a decline with retinotopic eccentricity. Moreover, my results suggested that the bandwidth of pSFT depends on eccentricity, and that populations with lower SF peaks possess broader bandwidths. In Experiment 2, I proposed a new visual task, coined the Numerosity Judgement Paradigm (NJP), for fine-grained parametric manipulation of attentional load. Eight healthy, young adults performed this task in an MRI scanner, and the analysis of the BOLD signal indicated that the activity within the putative dorsal attention network was precisely modulated as a function of the attentional load of the task. In Experiment 3, I used the NJP to modulate attentional load, and exploited the model-based approach to estimate pSFTs under different attentional states. fMRI results of 9 healthy, young adults did not reveal any changes in either peak or the bandwidth of the pSFTs with attentional load. This study yields a full visuocortical map of spatial frequency sensitivity and introduces a new paradigm for modulating attentional load. Although under this paradigm I did not find any changes in SF preferences within human visual areas with attentional load, I cannot preclude the possibility that changes emerge under different attentional manipulations.

<p> Although nearly one third of the Canadian population is projected to be over the age of 65 by the year 2030, we know relatively little about how aging affects brain function generally, let alone how aging affects visual perception. The current dissertation was conducted as part of a research programme designed to better characterize how aging affects visual perception. </p> <p> Older persons exhibit a variety of deficits for perception of complex visual forms. The perception of these complex forms-including everyday forms such as faces and objects-is subserved by low-level channels that are selective, or tuned, for the orientation and spatial frequency of luminance-defined contours in the visual scene. The bandwidth of these channels is inversely related to the amount of information that they can pass on to higher visual processes; narrowly-tuned channels are better. Single-cell physiological investigations of primates suggest that visual cortex neurons thought to subserve these channels exhibit broader tuning in senescence. If these channels become broadly-tuned in older aging, this could explain age-related deficits for complex form perception. In Chapters 2 and 3 of the current thesis, I measured the tuning of these channels in otherwise healthy, older humans using psychophysical masking techniques. In Chapters 4 and 5, I measured the average tuning of the neurons thought to underlie these channels in older human adults, physiologically, using electroencephalography (EEG). Despite the aforementioned reports of functional decline in senescent neurons, psychophysical and physiological orientation and spatial frequency tuning did not differ between younger and older adults. One explanation for this discrepancy is that there is a methodological issue in the single-cell primate literature wherein anesthetics interact with senescence to produce seemingly broader neural tuning. Another explanation is that older humans do have otherwise detuned neurons and channels, but are able to tune their neurons and channels by the action of consciousness, attention, or age-related compensatory brain reorganization. </p> / Thesis / Doctor of Philosophy (PhD)

Human visual perception is contingent on a visual system that can efficiently bridge the gap between perceived and physical reality, all while balancing the physical and metabolic constraints on processing. Early visual cortex has been identified as the foundation for this bridge, where modulatory signals, like attention and emotion, facilitate and bias processing towards the most relevant visual information for the task at hand. However, there remains an incomplete understanding of how visual processing is shaped by attention to time and features, and with emotional state. To address these gaps in knowledge, the experiments outlined in this dissertation use psychophysics, fMRI, and computational modeling to explore the mechanisms that subserve changes in perception due to attention- and emotion-based modulation of early visual cortex in humans. First, we examined the mechanisms that subserve temporal attention – the allocation of attention to a specific moment in time (Chapter 2). To do so, we manipulated temporal attention in an equivalent noise approach, measuring fine orientation discrimination thresholds in visual noise (n = 12). Under a normalization framework, results revealed that temporal attention selectively enhanced perceptual thresholds for a target signal across increasing noise levels. Moreover, a control experiment (n = 6) confirmed that temporal attention did not reduce uncertainty in target detection, but instead selectively enhanced the appearance of target information. Next, we examined how attention alters spatial frequency processing – the encoding of coarse and fine details in an image (Chapter 3). Previous studies in non-human primates have shown that attention modulates spatial frequency processing, but this remains untested in human early visual cortex. To do so, we leveraged an fMRI population spatial frequency tuning (pSFT) mapping approach while manipulating attention to spatial frequency (n = 8). Consistent with previous studies, attention altered spatial frequency processing across the visual field, but with respect to the attended spatial frequency and population receptive field eccentricity. Specifically, voxels with more peripheral receptive fields experienced an increase in preferred SF and a decrease in tuning bandwidth, while more foveal populations experienced a decrease in preferred SF and an increase in tuning bandwidth. These results support the spatial resolution hypothesis, suggesting that internal goals and required spatial resolution together bias visual processing. Lastly, we used the same fMRI pSFT mapping approach to test how emotional arousal alters spatial frequency processing in early human visual cortex (Chapter 4). Previous studies have found behavioral deficits in high spatial frequency processing in fearful contexts, where priority is given to lower spatial frequencies. However, there is a lack of human evidence supporting this change at the visuocortical population level. In our experiment (n = 7), we manipulated affective arousal with emotional sounds while concurrently mapping pSFT and recording physiological measures of arousal (pupillometry and skin conductance). Despite ambiguous physiological measures of arousal, we found a significant and non-monotonic effect of sound on pSFT that varied as a function of spatial frequency preference – a pattern of results that mirrors how attention modulates spatial frequency processing. Together, these findings speak to the malleability of the human visual system in selectively altering visual processing with respect to attentional and emotional cues. Chapter 1 provides an important insight into how attention to time selectively enhances visual target signals in increasingly noisy environments. Additionally, Chapters 2 and 3 are the first to provide quantitative measures for how attention and arousal alter spatial frequency processing in early visuocortical populations, contributing to a deeper understanding of the adaptability of the human visual system to internal and environmental demands.

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Yes / In this study, we investigated whether reading influences contrast adaptation differently in young adult emmetropic and myopic participants at the spatial frequencies created by text rows and character strokes. Pre-adaptation contrast sensitivity was measured for test gratings with spatial frequencies of 1cdeg-1 and 4cdeg-1, presented horizontally and vertically. Participants then adapted to reading text corresponding to the horizontal “row frequency” of text (1cdeg-1), and vertical “stroke frequency” of the characters (4cdeg-1) for 180s. Following this, post-adaptation contrast sensitivity was measured. Twenty young adults (10 myopes, 10 emmetropes) optimally corrected for the viewing distance participated. There was a significant reduction in logCS post-text adaptation (relative to pre-adaptation logCS) at the row frequency (1cdeg-1 horizontal) but not at the stroke frequency (4cdeg-1 vertical). logCS changes due to adaptation at 1cdeg-1 horizontal were significant in both emmetropes and myopes. Comparing the two refractive groups, myopic participants showed significantly greater adaptation compared to emmetropic participants. Reading text on a screen induces contrast adaptation in young adult observers. Myopic participants were found to exhibit greater contrast adaptation than emmetropes at the spatial frequency corresponding to the text row frequency. No contrast adaptation was observed at the text stroke frequency in either participant group. The greater contrast adaptation experienced by myopes after reading warrants further investigation to better understand the relationship between near work and myopia development.

Yes / We investigated whether changes in low-level image characteristics, in this case spatial frequency, were capable of generating a well-known expansion in the perceived duration of an infrequent “oddball” stimulus relative to a repeatedly-presented “standard” stimulus. Our standard and oddball stimuli were Gabor patches that differed from each other in spatial frequency by two octaves. All stimuli were equated for visibility. Rather than the expected “subjective time expansion” found in previous studies, we obtained an equal and opposite expansion or contraction of perceived time dependent upon the spatial frequency relationship of the standard and oddball stimulus. Subsequent experiments using equi-visible stimuli reveal that mid-range spatial frequencies (ca. 2 c/deg) are consistently perceived as having longer durations than low (0.5 c/deg) or high (8 c/deg) spatial frequencies, despite having the same physical duration. Rather than forming a fixed proportion of baseline duration, this bias is constant in additive terms and implicates systematic variations in visual persistence across spatial frequency. Our results have implications for the widely cited finding that auditory stimuli are judged to be longer in duration than visual stimuli. / Wellcome Trust, UK, the Federation of Ophthalmic and Dispensing Opticians, UK, and the College of Optometrists, UK.

The effect of refractive blur upon postural stability was investigated under three conditions: normal standing, standing with input from the somatosensory system disrupted and standing with input from the somatosensory and vestibular systems disrupted. Standing stability was assessed using the centre of pressure (COP) signal from force plate data in four young subjects (mean 23.9 ± 3.1 years) and five repeated sets of measurements were taken. The subjects looked straight ahead at a horizontal and vertical square wave pattern of 2.5 cycles (degree)¿1. Under each of the three test conditions, standing stability was measured with the optimal refractive correction and under binocular blur levels of 0, + 1, + 2, + 4, and + 8 D and with eyes closed. In the normal standing condition, dioptric blur had only a mild effect on postural stability. However refractive blur produced large increases in postural instability when input from one or both of the other two sensory systems were disrupted. We hypothesized that dioptric blur would have an even great effect on postural stability if the visual target used was of higher spatial frequency. This was confirmed by repeated measurements on one subject using a target of 8 cycles (degree)¿1. The study highlights the possible importance of an optimal correction to postural stability, particular in situations (or people) where input from the somatosensory and/or vestibular systems are disrupted, and where the visual surrounds are of high spatial frequency.

Master of Science / Department of Psychology / Lester C. Loschky / To investigate the role of various bands of spatial frequencies for drawing, untrained artists drew four portraits from four different bands of spatial frequencies (e.g. unfiltered, 4-8, 8-16, & 16-32 cycles per face width (c/fw)). Raters then judged the accuracy of the drawings in comparison to both the source image from which the drawings were produced and an unfiltered version of the same face. The results show that low spatial frequencies (LSFs) and high spatial frequencies (HSFs) were useful for drawing, relative to middle spatial frequencies (MSFs). Additionally, the unfiltered condition that contained all spatial frequencies produced the most accurate drawings. This suggests that when artists are allowed access to both LSFs and HSFs they are able to utilize the global structure information carried in LSFs as well as the edge and detail information carried in HSFs to create more accurate drawings. The author posits that the MSFs that are useful for face recognition become redundant for drawing and that novice artists discount these MSFs in the control condition in order to increase the saliency and usefulness of the LSFs and HSFs. The results have implications for art education, drawing technology and the development of low-level drawing theory.

As optical designs become more sophisticated and incorporate aspheric and free form surfaces, the need to specify limits on mid-spatial frequency manufacturing errors becomes more critical, particularly as we better understand the effects of these errors on image quality. While there already exist methods based on Fourier analysis to specify these errors in most commercial interferometry software, the method of calculation and the power spectral density (PSD) results remain obscure to many in the optical design and manufacturing field. We suggest that the structure functions (SF) contains the same information as in the Fourier based PSD but in a way that is much more transparent to analysis, interpretation and application as a specification. The units of measure are more familiar and the concept behind the analysis is simpler to understand. Further, the information contained in the structure function (or PSD) allows a complete specification of an optical surface from the finest measurable detail of roughness to the overall figure. We discuss the origin of the structure function in the field of astronomy to describe the effects of air turbulence on image quality, the simple mathematical definition of the structure function and its easy means of calculation and how its results should be scaled depending on the location of the optical surface in a system from pupil to image plane. Finally, we give an example of how to write a specification of an optical surface using the structure function.

O presente trabalho teve por objetivo pesquisar os efeitos que as bandas de freqüência espacial alta e baixa têm no reconhecimento de faces em campo visual lateralizado. Foram distribuídos aleatoriamente 40 participantes em dois grupos. Os voluntários observaram 14 fotos de faces sem filtragem até memorizá-las. A seguir foram apresentadas 56 fotos de faces com filtragens de freqüências espaciais, intercaladas aleatoriamente com apresentações de faces não mostradas anteriormente. Cada uma delas foi exibida na tela durante 300 ms mediante a metodologia de apresentação dicótica. O participante devia responder se a face mostrada pertencia ao grupo de fotos inicialmente observado. As freqüências de respostas permitiram calcular as curvas ROC (Receiver Operating Characteristic) e os parâmetros Az e da preconizado pela Teoria de Detecção de Sinal (MACMILLAN; CREELMAN, 2005) para as faces naturais, faces compostas de freqüências espaciais baixas e faces compostas de freqüências espaciais altas. Os resultados obtidos mostram que as faces Originais foram melhor reconhecidas do que as faces com Freqüências Espaciais Altas (FEA) ou Freqüências Espaciais Baixas (FEB). Ao contrário do achado na literatura, o Hemisfério Esquerdo (HE) teve uma tendência a reconhecer mais eficazmente as faces do que o Hemisfério Direito (HD), independente da condição de filtragem. O HD é igualmente competente do que o HE para processar FEB, mas pior do que o HE para processar FEA. Quanto à performance por gênero, tanto homens quanto mulheres tiveram um desempenho similar quando as faces foram processadas com o HD. O desempenho das mulheres ficou de acordo com a hipótese da FE, pois o reconhecimento que fizeram para as faces com FEA foi melhor do que para as que tinham predomínio de FEB. Os homens, mesmo com o HE, fizeram um reconhecimento melhor das faces com predomínio de FEB do que das faces com FEA. / This study was made with the objective of investigate the effects of high and low spatial frequency bands in face recognition in lateralized visual field. 40 participants were randomly distributed in two groups. The volunteers viewed fourteen non-filtered pictures of faces until they managed to memorize them. After that, fifty six spatial frequency filtered pictures of faces were presented randomly interspersed with pictures of faces previously showed. Each one of them was exhibited in the screen for three hundred milliseconds using the dichotic presentation procedure. The participant should answer whether the face presented belonged to the group of pictures initially viewed. The frequency of responses allowed to calculate the ROC (Receiver Operating Characteristic) Curves and the Az and da parameters praised by the Signal Detection Theory (Macmillan; Creelman, 2005) for natural faces, low spatial frequency composed faces and high spatial frequency composed faces. Results showed that original faces were better recognized than faces with high spatial frequencies (HSF) and low spatial frequencies (LSF). Differently from literature, the left hemisphere was more accurate than the right to recognize faces, regardless of the filter condition. The RH was equivalent to the LH to process LSF, but worse than the LH to process HSF. Concerning the performance of the genders, men and women judged faces in a very similar way when they used the RH. The performance of women agreed with the FE hypothesis, being faces with HSF recognized better than faces with LSF. Men, even using the LH, were more accurate to recognize faces with LSF than HSF.