Biomedical subjects

Charles A Collin

Publications and source records attributed to Charles A Collin.

7 recordsLinked to original sources

PubMed2006

Spatial-frequency thresholds for object categorisation at basic and subordinate levels.

In an attempt to understand how low-level visual information contributes to object categorisation, previous studies have examined the effects of spatially filtering images on object recognition at different levels of abstraction. Here, the quantitative thresholds for object categorisation at the basic and subordinate levels are determined by using a combination of the method of adjustment and a match-to-sample method. Participants were asked to adjust the cut-off of either a low-pass or high-pass filter applied to a target image until they reached the threshold at which they could match the target image to one of six simultaneously presented category names. This allowed more quantitative analysis of the spatial frequencies necessary for recognition than previous studies. Results indicate that a more central range of low spatial frequencies is necessary for subordinate categorisation than basic, though the difference is small, at about 0.25 octaves. Conversely, there was no effect of categorisation level on high-pass thresholds.

Adult↗

PubMed2006

Spatial frequency thresholds for face recognition when comparison faces are filtered and unfiltered.

Previous work has shown an advantage of middle spatial frequencies (SFs) in face recognition. However, a few recent studies have suggested that this advantage is reduced when comparison and test stimuli are spatially filtered in a similar way. In the present study, we used standard psychophysical methods, in combination with a match-to-sample task, to determine the SF thresholds for face matching under conditions in which: (1) comparison stimuli were unfiltered and (2) comparison stimuli were spatially filtered in the same way as test stimuli. In two experiments, we show that SFs closer to the middle band are sought out more in the former case than in the latter. These results are compatible with the idea that a middle band of SFs will be most useful for any visual task and that the breadth of this optimal middle band will vary depending on task characteristics.

Adult↗

PubMed2005

Recognizing faces defined by texture gradients.

Texture gradients can reveal surface orientation in a manner similar to shape from shading, and therefore provide an important cue for object recognition. In this study, we tested whether a complex 3-D object, such as a face, can be identified from texture gradients alone. The stimuli were laser-scanned faces for which the texture element was a fractal-noise pattern mapped onto the 3-D surface. An eight-alternative forced choice task was used in which participants matched a face defined by texture gradients to one of eight faces defined by shape from shading (Experiment 1) or by texture gradients (Experiment 2). On average, participants scored 24% and 18%, respectively, above chance in these experiments. Although this performance was much poorer than the performance based entirely on shape-from-shading stimuli (Experiment 3), the results suggest that texture gradient information may be used to recover surface geometry of complex objects.

Contrast Sensitivity↗

PubMed2005

Subordinate-level categorization relies on high spatial frequencies to a greater degree than basic-level categorization.

In two experiments, category verification of images of common objects at subordinate, basic, and superordinate levels was performed after low-pass spatial filtering, high-pass spatial filtering, 50% phase randomization, or no image manipulation. Both experiments demonstrated the same pattern of results: Low-pass filtering selectively impaired subordinate-level category verification, while having little to no effect on basic-level category verification. Subordinate categorization consequently relies to a greater degree on high spatial frequencies of images. This vulnerability of subordinate-level processing was specific to a lack of high spatial frequency information, as opposed to other visual information, since neither high-pass filtering nor the addition of phase noise produced a comparable reduction in performance. These results are consistent with the notion that object recognition at basic levels relies on the general shapes of objects, whereas recognition at subordinate levels relies on finer visual details.

Adult↗

PubMed2004

Face recognition is affected by similarity in spatial frequency range to a greater degree than within-category object recognition.

Previous studies have suggested that face identification is more sensitive to variations in spatial frequency content than object recognition, but none have compared how sensitive the 2 processes are to variations in spatial frequency overlap (SFO). The authors tested face and object matching accuracy under varying SFO conditions. Their results showed that object recognition was more robust to SFO variations than face recognition and that the vulnerability of faces was not due to reliance on configural processing. They suggest that variations in sensitivity to SFO help explain the vulnerability of face recognition to changes in image format and the lack of a middle-frequency advantage in object recognition.

Adolescent↗

PubMed2002

Using Matlab to generate families of similar Attneave shapes.

We present a program for Matlab that quickly generates Attneave-style random polygons and families of similar polygons. The function allows a great deal of user control over various aspects of the shape generation process. It also has the ability to detect and eliminate shapes that do not match a variety of user-entered parameters regarding the lengths of the shapes' sides, vertex angles, and topological form. The function eliminates the time-consuming task of generating such shapes by hand and should allow their broader use in behavioral research. The Matlab script function can be downloaded at www.dal.ca/~mcmullen/downloads.html.

Algorithms↗

PubMed2002

Using MATLAB with the psychophysics toolbox to present the heterochromatic fusion nystagmus stimulus.

We present a program for MATLAB that generates and presents the heterochromatic fusion nystagmus stimulus. This stimulus allows assessment of isoluminant states through recordings of reflexive eye movements (optokinetic nystagmus). The reflexive nature of the subject's response makes this stimulus especially useful with nonverbal subjects, such as children and animals. Unfortunately, the stimulus is complex and difficult to program. By presenting the present program, we hope to help those who wish to use this tool in their research. The code of the function can be downloaded at www. dal.ca/-mcmullen/downloads.html.

Eye Movements↗