Philip Servos

Wilfrid Laurier University, Waterloo, Ontario, Canada

Are you Philip Servos?

Claim your profile

Publications (60)167.33 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: Recent research has suggested the existence of a modality-independent memory system that is responsible for storing representations of simple, scalar stimulus attributes, such as the frequency of an auditory pure tone or the duration of a stimulus. In the present study, we modify an existing computational model of short-term memory (STM) for stimulus frequency to allow it to perform STM tasks for both stimulus frequency and stimulus duration, supporting the notion of a common scalar STM system. We further demonstrate the utility of the model by showing that it can reproduce the subjective shortening effect, a classic finding in the psychophysical literature. (PsycINFO Database Record (c) 2014 APA, all rights reserved).
    Canadian Journal of Experimental Psychology 12/2014; 68(4):236-41. · 1.02 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In a previous study, Harris et al. (2002) found disruption of vibrotactile short-term memory after applying single-pulse transcranial magnetic stimulation (TMS) to primary somatosensory cortex (SI) early in the maintenance period, and suggested that this demonstrated a role for SI in vibrotactile memory storage. While such a role is compatible with recent suggestions that sensory cortex is the storage substrate for working memory, it stands in contrast to a relatively large body of evidence from human EEG and single-cell recording in primates that instead points to prefrontal cortex as the storage substrate for vibrotactile memory. In the present study, we use computational methods to demonstrate how Harris et al.'s results can be reproduced by TMS-induced activity in sensory cortex and subsequent feedforward interference with memory traces stored in prefrontal cortex, thereby reconciling discordant findings in the tactile memory literature.
    Frontiers in Computational Neuroscience 03/2014; 8:23. · 2.23 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Traditionally, working and short-term memory (WM/STM) have been believed to rely on storage systems located in prefrontal cortex (PFC). However, recent experimental and theoretical efforts have suggested that, in many cases, sensory or other task-relevant cortex is the actual storage substrate for WM/STM. What factors determine whether a given WM/STM task relies on PFC or sensory cortex? In the present article, we outline recent experimental findings and suggest that the dimensionality or complexity of the to-be-remembered property or properties of a stimulus can be a determining factor.
    Attention Perception & Psychophysics 01/2014; · 1.97 Impact Factor
  • Source
    Neuroscience Letters 06/2013; 544:163. · 2.06 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The encoding of irrelevant stimuli into the memory store has previously been suggested as a mechanism of interference in working memory (e.g., Lange & Oberauer, Memory, 13, 333-339, 2005; Nairne, Memory & Cognition, 18, 251-269, 1990). Recently, Bancroft and Servos (Experimental Brain Research, 208, 529-532, 2011) used a tactile working memory task to provide experimental evidence that irrelevant stimuli were, in fact, encoded into working memory. In the present study, we replicated Bancroft and Servos's experimental findings using a biologically based computational model of prefrontal neurons, providing a neurocomputational model of overwriting in working memory. Furthermore, our modeling results show that inhibition acts to protect the contents of working memory, and they suggest a need for further experimental research into the capacity of vibrotactile working memory.
    Cognitive Affective & Behavioral Neuroscience 11/2012; 13(1). · 3.87 Impact Factor
  • Source
    Philip Servos, Allison Boyd
    [Show abstract] [Hide abstract]
    ABSTRACT: Using the touch-induced visual illusion we examine whether the brain regions involved in coding sensory information are dissociable from those that contain decision information. Activity in the intraparietal sulcus, as measured by functional magnetic resonance imaging, was associated with the illusion suggesting a sensory coding role whereas activity in the middle occipital gyrus differentially modulated activity according to the decisions made by subjects consistent with their reported perceptual phenomenology.
    PLoS ONE 10/2012; 7(10):e47788. · 3.53 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The nature of interference in working memory has been a subject of discussion for decades. It has previously been argued that irrelevant stimuli can interfere with working memory by being encoded into memory. Previous findings have suggested that irrelevant sensory activity can interfere with the storage of information in tactile working memory. More recently, it has been suggested that this type of interference may operate through the overwriting of stored information by interfering sensory stimuli, even when participants are instructed to ignore such stimuli. Such a mechanism of interference is consistent with previous theoretical proposals. In the present study, we use a computational diffusion model to demonstrate that previous empirical findings are best explained by the encoding of irrelevant sensory information and subsequent interference.
    Neuroreport 03/2012; 23(4):255-8. · 1.40 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Vibrotactile working memory is increasing in popularity as a model system to test theories of working memory. Notably, however, we know little about vibrotactile working memory capacity. While most other domains of working memory are able to store multiple items (for example, the seven-plus-or-minus-two capacity of verbal memory [17]), previous examinations of vibrotactile working memory suggest that stored items may suffer from high levels of interference in the form of overwriting or representation-based interference [2,4], potentially limiting capacity and also limiting our ability to draw comparisons between vibrotactile working memory and other forms of working memory. In the present study, we use a two-item delayed match-to-sample paradigm to demonstrate that subjects are able to store multiple items in vibrotactile working memory, suggesting that interference does not catastrophically limit capacity, and strengthening our ability to compare vibrotactile working memory to other working memory tasks.
    Neuroscience Letters 02/2012; 514(1):31-4. · 2.06 Impact Factor
  • Source
    Peggy J Planetta, Philip Servos
    [Show abstract] [Hide abstract]
    ABSTRACT: The tactile motion aftereffect (tMAE) is a perceptual illusion in which a stationary stimulus feels as though it is moving when presented following adaptation to a unidirectionally moving tactile stimulus. Using functional magnetic resonance imaging (fMRI), we localized the brain areas responsive to tactile motion and then investigated whether these areas underlie the tMAE. Tactile stimulation was delivered to the glabrous surface of the right hand by means of a plastic cylinder with a square-wave patterned surface. In the tactile motion localizer, we contrasted periods in which the cylinder rotated at 15 rpm with periods of rest (stationary contact). Activation was observed in the contralateral (left) thalamus, postcentral gyrus, and parietal operculum. In the tMAE experiment, the cylinder rotated at 15 or 60 rpm for 2 min. The 60-rpm speed induced reliable tMAEs, whereas the 15-rpm speed did not. Of the areas activated by the tactile motion localizer, only the postcentral gyrus showed a sustained fMRI response following the offset of 60-rpm (but not 15-rpm) stimulation, presumably reflecting the illusory perception of motion.
    Experimental Brain Research 11/2011; 216(4):535-44. · 2.17 Impact Factor
  • Source
    Psychonomic Society Meeting; 11/2011
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: In previous studies of interference in vibrotactile working memory, subjects were presented with an interfering distractor stimulus during the delay period between the target and probe stimuli in a delayed match-to-sample task. The accuracy of same/different decisions indicated feature overwriting was the mechanism of interference. However, the distractor was presented late in the delay period, and the distractor may have interfered with the decision-making process, rather than the maintenance of stored information. The present study varies the timing of distractor onset, (either early, in the middle, or late in the delay period), and demonstrates both overwriting and non-overwriting forms of interference.
    PLoS ONE 07/2011; 6(7):e22518. · 3.53 Impact Factor
  • Source
    Philip Servos, Mounia Ziat
    International Society for Behavioural Neuroscience (ISBN); 06/2011
  • Source
    Cognitive Neuroscience Society (CNS); 04/2011
  • Tyler Bancroft, Philip Servos
    [Show abstract] [Hide abstract]
    ABSTRACT: We use a vibrotactile-delayed match-to-sample paradigm to evaluate the effects of interference on working memory. One of the suggested mechanisms through which interference affects performance in working memory is feature overwriting: Short-term representations are maintained in a finite set of feature units (such as prefrontal neurons), and distractor stimuli co-opt some or all of those units, degrading the stored representation of an earlier stimulus. Subjects were presented with two vibrotactile stimuli and were instructed to determine whether they were of the same or different frequencies. A distractor stimulus was presented between the target and probe stimuli, the frequency of which was a function of the target stimulus. Performance on the task was affected by the frequency of the distractor, with subjects making more erroneous same judgments on different trials when the distractor frequency was closer to the probe than to the target, than when the distractor was further from the probe than the target. The results suggest that the frequency of the distractor partially overwrites the stored frequency information of the probe stimulus, providing support for the feature-overwriting explanation of working memory interference.
    Experimental Brain Research 02/2011; 208(4):529-32. · 2.17 Impact Factor
  • Source
    Frontiers in Human Neuroscience 01/2011; 5:162. · 2.90 Impact Factor
  • Journal of Vision 12/2010; 1(3):483-483. · 2.73 Impact Factor
  • Canada Research Chair; 11/2010
  • Source
    Peggy J Planetta, Philip Servos
    [Show abstract] [Hide abstract]
    ABSTRACT: We investigated the effect of adapting speed, duration, and distance on the frequency of occurrence, duration, and vividness of the tactile motion aftereffect (tMAE). Using a cylindrical drum with a patterned surface we adapted the glabrous surface of the right hand at two speeds (14 and 28 cm/s) and three durations (60, 120, and 240 s). Distance was explored in the interaction of adapting speed and duration. The results showed that the frequency of occurrence, duration, and vividness of the tMAE increased with adapting speed. There was also a positive relationship between adapting duration and the frequency of occurrence, but not the duration or vividness, of the illusion. Distance was only a factor when it came to the duration of the tMAE. Taken together, these results show the importance of adapting parameters, particularly speed, on the tMAE.
    Somatosensory & Motor Research 09/2010; 27(3):100-5. · 0.58 Impact Factor
  • Journal of Vision 06/2010; 6(6):177-177. · 2.73 Impact Factor
  • Source
    Peggy J Planetta, Philip Servos
    [Show abstract] [Hide abstract]
    ABSTRACT: The motion aftereffect (MAE) refers to the apparent motion of a stationary stimulus following adaptation to a continuously moving stimulus. There is a growing consensus that the fast adapting (FA) rather than the slowly adapting (SA) afferent units mediate the tactile version of the MAE. The present study investigated which FA units underlie the tactile MAE by measuring its prevalence, duration, and vividness on different skin areas that vary in their composition of FA units. Specifically, the right cheek, volar surface of the forearm, and volar surface of the hand were adapted using a ridged cylindrical drum, which rotated at 60 rpm for 120 s. Although there was no difference in duration or vividness between the skin surfaces tested, the tactile MAE was reported twice as often on the hand compared to the cheek and forearm, which did not differ significantly from one another. This suggests that the FA I units in the glabrous skin and the hair follicle and/or the FA I and field units in the hairy skin contribute to the tactile MAE.
    Experimental Brain Research 04/2010; 202(2):377-83. · 2.17 Impact Factor

Publication Stats

1k Citations
167.33 Total Impact Points

Institutions

  • 1998–2013
    • Wilfrid Laurier University
      • Department of Psychology
      Waterloo, Ontario, Canada
  • 2011
    • University of Illinois at Chicago
      • Department of Kinesiology and Nutrition
      Chicago, IL, United States
  • 2008
    • Massachusetts Institute of Technology
      • Department of Brain and Cognitive Sciences
      Cambridge, MA, United States
  • 1992–2002
    • The University of Western Ontario
      • Department of Psychology
      London, Ontario, Canada
  • 1995
    • Stanford University
      • Department of Psychology
      Stanford, CA, United States
  • 1990
    • University of Guelph
      • Department of Psychology
      Guelph, Ontario, Canada