Functional MRI correlates of category and parity judgements about rotated letters

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Mental rotation and the parietal cortex

Mental rotation is considered to be a prototypical example of a visuospatial cognitive process (Corballis, 1997). As indicated earlier, according to Ungerleider and Mishkin’s (1982) model, the dorsal visual stream, culminating in the PPC subserves visual perception of spatial properties of an image. If this is an accurate interpretation of the functional subdivision within the visual system, then it seems reasonable that mental rotation, which depends on imagined spatial transformation of a mental image, would depend on the dorsal visual system, and specifically the PPC. The involvement of the PPC in parity tasks has been supported by a number of other neuroimaging studies (Cohen et al., 1996; Harris et al., 2000; Jordan et al., 2001; Jordan, Schadow, Wuestenberg, Heinze, & Jäncke, 2004; Jordan, Wüstenberg, Heinze, Peters, & Jäncke, 2002; Koshino et al., 2005; Lamm et al., 2001; Podzebenko et al., 2002; Richter et al., 1997; Tagaris et al., 1996, 1997; Tagaris et al., 1998), as well as neuropsychological (Ditunno & Mann, 1990; Farah & Hammond, 1988; Mehta & Newcombe, 1991), electrophysiological (Bajric et al., 1999; Heil, 2002; Heil et al., 1998.

Mental rotation and the prefrontal cortex

Prefrontal activation is commonly attributed to working memory or attentional demands (Carpenter et al., 1999; Cohen et al., 1996; Podzebenko et al., 2002). However, the temporal characteristics of the BOLD signal are comparable between the higher-order premotor areas and PPC. For example, Richter et al. (2000) have shown that the duration of the BOLD signal increases within higher-order pre-motor regions is correlated with RTs, comparable to the effects observed within the PPC (Richter et al., 1997). Furthermore, Lamm et al. (2001) have shown that parietal and pre-motor activation follow similar time-course. It is, nevertheless possible that higher-order pre-motor activation may not reflect mental rotation per se despite the correlations with the RTs and PPC activation. Koshino et al. (2005) found that increasing stimulus complexity, and thus increasing attentional and working memory load, elicits increases in activation in these areas as a function of angular orientation. Furthermore, frontal lobectomy of either the left or the right prefrontal cortex apparently does not impair mental rotation (Alivisatos, 1992).

Functional MRI correlates of category and parity judgements about

rotated letters and digits3 Introduction In this study, neural activity was monitored using functional MRI while participants performed parity, or normal-backward, judgements and letter-digit category judgements about rotated alphanumeric characters. The typical finding is that parity judgements elicit steady increases in RTs with angular departure from upright (Cooper & Shepard, 1973). This increase in RTs is thought to reflect use of mental-rotation strategy whereby the participants imagine a mental image rotating from the position in which the stimulus was originally presented to the canonical upright orientation of the character. Subsequently, the decision regarding its parity is made by comparing the mental image to a memory representation of the character (Cooper & Shepard, 1973). On the other hand, classification of the characters as letters or digits does not require mental rotation and RTs do not systematically depend on orientation of the stimuli (Corballis & Nagourney, 1978).

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Mental rotation

Linear increases in RTs and the BOLD signal as a function of angular orientation were found only observed in response to the parity task. Linear increases in the RTs for parity tasks are commonly associated with mental rotation. I hypothesised that comparable effects should be seen in the BOLD signal originating in the areas that are involved in mental rotation per se. The observation that linear effects were only observed in the parity task provides additional support for the idea that linear increases in the BOLD signal are indeed neural correlates of mental rotation. Only a subset of areas involved in the parity judgement task showed linear increases in BOLD activation with larger angular orientations. The mental-rotation related effects were restricted to a subregion of the IPS – the middle portion of the dorsal IPS 75 (dIPSm), and specific prefrontal areas – the lateral and the medial pre-SMA. These findings support the notion that mental rotation is subserved by both parietal and higher-order premotor regions.

Table of contents :

Abstract
Acknowledgements
Table of contents
List of Figures
List of Tables
List of Abbreviations
List of Conference Abstracts
Chapter 1: General Introduction
- Perceptual effects of angular orientation
- Changes in orientation and object recognition
- Orientation-dependent neural processing
- Mirror-image equivalence, parity discrimination and mental rotation
- The present thesis
Chapter 2: Functional MRI correlates of category and parity judgements about rotated letters
- and digits
- Introduction
- Methods
- Results
- Discussion
Chapter 3: ERP correlates of misoriented object processing during category and parity
- judgements
- Introduction
- Methods
- Results
- Discussion
Chapter 4: ERP correlates of stimulus-orientation effects in a letter-number categorisation
- task, and a colour-judgement task
- Introduction
- Methods
- Results
- Discussion
Chapter 5: General discussion
- Neuroanatomical Correlates: fMRI analysis
- Sequential components: EEG analysis
- Mental rotation
- Summary and Conclusion
- References