Our ability to detect, discriminate, and identify objects, to extract depth information through the use of stereopsis, and to learn about and classify different surface properties and textures are all secondary to first extracting useful cues about the local contrast within a visual scene. Contrast can be made by a change in a variety of visual features. We will specifically consider the contrast made by first-order and second-order luminance changes, specifically the contrast between local pixel luminance, and the contrast between local pixel luminance variance. There are a variety of tools available to `observe' cortical modulation to contrast, such as electrophysiological recordings of spike rate or local field potentials, fMRI BOLD modulation, magnetic encephalography (MEG), or visually evoked potentials (VEP), or through behavioral markers such as detection thresholds and discrimination thresholds. BOLD fMRI modulation provides a unique tool to measure the early visual response to local changes in image contrast with superior spatial specificity and minimal subject invasiveness, while psychophysics allows us to quantify the information human observers use to detect contrast. We will use both BOLD fMRI and psychophysics to explore three components of the human response to contrast: 1) how much do we use contrast when it forms the edge of a target shape compared to when it composes the interior area of the shape; 2) how well does second-order contrast modulate early visual cortex compared to the modulation elicited by first-order contrast; and 3) how accurate is the timing information of functional magnetic resonance imaging (fMRI) of the blood oxygen-level dependent (BOLD) modulation.
Human observers do not always use contrast in the same way, and in the first experiment we study how human observers use first-order contrast in an image region versus second-order contrast at a region boundary when accomplishing a difficult detection task. We may expect V1 response to differ when contextual cues suggest that contrast is contained within an object border compared to when it fills in the interior texture of an object. Consider the case of two Holstein (black and white spotted) cows standing together, one partially occluding the other. We can use contrast cues to both identify which parts belong to which cow, as well as to determine the type of cow based on the spotted character of its hide. Both the segmentation task and the identification task require use of the same type of contrast information. However, optimal processing may change the way that low-level cues are handled in early visual cortex by modulation through context-dependent feedback as well learned information about cows. This may give rise to similar low-level cues producing different neural signals in early cortical areas, and subsequently, different sensitivity to cues in a border compared to a region. Our first experiment explores observers' use of contrast contained within the edge of a detectable target compared to the contrast that makes up the entire interior region of the target.
Extraction of local image contrasts occurs at an early stage of visual processing, however, which types of contrast and how strongly they modulate early visual cortex remains undetermined. There are a variety of contrasts that could be compared. The first-order versus second-order contrast comparison is appealing because, with images generated by locally changing either average pixel luminance or pixel variance in white noise, we can create stimuli that are equal in orientation and spatial frequency information, contain detectable boundaries, and still require independent information to be extracted for detection. Our second experiment quantifies the response in primary visual cortex (V1) to first- and second-order using fMRI to acquire a spatially specific edge response in human subjects.
Finally, it would be ideal to acquire both spatially and temporally specific measurements of cortical modulation in response to contrast changes. Several research groups around the world have begun using the relative timing between BOLD fMRI events to assign causal significance to the temporal order of modulation across different cortical areas using dynamic causal modeling. If, however, the timing of the modulation depends on the strength of the modulation, we could draw false conclusions from relative timing comparisons. Therefore, in our third experiment we measured three temporal characteristics of the BOLD hemodynamic response function (HRF) as a function of stimulus contrast: onset latency, time to peak, and full-width half-maximum.
These explorations into the human and BOLD fMRI response to contrast are aimed at developing the current knowledge about how we perceive and parse the world around us, as well as how we can better interpret one of our measurement tools, fMRI, as a correlate of the neuronal activity occurring in early visual cortex. Both aims guide us toward a better understanding of the mechanisms we employ to process the rich information our visual systems acquire.
University of Minnesota. Ph.D. dissertation. January 2010. Major: Neuroscience. Advisors:Daniel Kersten and Cheryl Olman. 1 computer file (PDF); ix, 89 pages, Ill. (some col.)
Thompson, Serena Kainoa Au.
Temporal and spatial properties of the BOLD fMRI response to first and second order contrast in V1..
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