Seed production in tall fescue (Festuca arundinacea Schreb.) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Seed Technology at Massey University, Palmerston North, New Zealand
The Paced Auditory Serial Addition Task ( PASAT ) was lateralised for the purpose of investigation into hemispheric specialisation for temporal integration. A right ear advantage ( REA ), representing a left hemisphere ( LH ) advantage in normal participants, was predicted due to the sequential nature of the PASAT, the previous finding of a REA by Norman ( 1984 ), and Miller's ( 1996 ) theory on the LH specialisation for delayed axonal conduction. The REA was also expected given other abundant evidence on LH superiority for the processing of temporal information ( Bradshaw & Nettleton, 1981; Bryson, Mononen, & Yu, 1980; Elfgren & Risberg, 1998; Gordon, 1978; Leek & Brandt, 1983; Prior, Kinsella, & Giese, 1990; Robinson, & Solomon, 1974; Troyer, Moscovitch, Winocur, Alexander, & Stuss, 1998 ). However, no REA was found in the present research. Careful precision in randomising and counterbalancing arithmetic outcomes, removal of the digit seven because of its two-syllable pronunciation, and the randomisation of ear of presentation and stimulus presentation rate, possibly eliminated procedural artefacts that were present in Norman's ( 1984 ) study. Therefore, despite the sequential nature of the PASAT it now appears to lack efficacy for research into temporal integration. Other findings from the present research instead showed support for the hypotheses of the right hemisphere's ( RH ) specialisation for vigilance ( Coslett, Bowers, & Heilman, 1987; Dimond, 1979; Heilman & van den Abell, 1979, 1980; Howes & Boller, 1975; Ladavas, Del Pesce, Mangun, & Gazzaniga, 1994; Ladavas, Del Pesce, & Provinciali, 1989; Loring, Meador, & Lee, 1989; Pardo, Fox, & Raichle, 1991; Simos & Molfese, 1997; Whitehead, 1991; Wilkins, Shallice, & McCarthy, 1987; Yokohama et al., 1987 ). For normal participants a left ear advantage ( LEA ) was found and more clearly demonstrated with an unpredictable task condition and with the faster presentation rates ( 1.2 & 2.0 s ) of the lateralised PASAT. The LEA was shown with both the same ear stimulus pairings ( LL ) and the left then right pairings ( LR ). Inferior performance with the right then left ( RL ) stimulus pairings was found in the unpredictable task condition and another task condition in which the side of stimulus presentation was predictable. Clinical participants (i.e., participants with a history of schizophrenia ) did not demonstrate a LEA, and they showed no difference compared to normal controls in performance with the right ear ( LH ). However, the clinical participants did manifest a particular disadvantage when stimulus processing required a shift from left to right ear ( LR ), but not the reverse ( RL ). The clinical participants manifested the LR deficit in the unpredictable and predictable task conditions. This LR disadvantage bears some resemblance to a previous finding, using the Visual Reaction Time Task, of a lateralised deficit in disengaging attention from the left visual field. Some researchers ( Bustillo et al., 1997; Posner, Early, Reiman, Pardo, & Dhawan, 1988; Posner & Raichle, 1994; Wigal, Swanson, & Potkin, 1997 ) interpret this LR deficit as a problem of slowness of responding by the LH ( right visual field ) after having attention oriented to the opposite field. However, the findings in the present research of a LEA for normal controls (i.e., LL and LR ), a relative deficit with RL for normals, and no difference in right ear ( RR ) performance between controls and clinical participants, and notably with the unpredictable condition, suggests an alternative interpretation. The LR deficit in schizophrenia may instead be due to difficulty in disengaging from the left visual or auditory field rather than impaired performance with the right field. The LR deficit may be as a result of dysfunction of vigilance, which is normally attributed to greater proficiency of the right hemisphere. In addition, opposing trends were observed for the clinical participants compared to the normal participants at particularly slow presentation rates, and with the predictable task condition in which stimuli were presented singularly to each hemisphere. Clinical participants showed a worsening of performance with the task. Normal participants demonstrated better performance with this task compared to another task condition, much like the standard PASAT, in which both ears received a stimulus simultaneously. These trends reflect Hellige's ( 1987, 1993 ) model of cross-hemispheric integration in which it is hypothesised that for normal participants single hemisphere processing has some advantage with lighter tasks. These trends also reflect the possibility that people with schizophrenia, in slow stimulus conditions, have difficulty whereby they unnecessarily over engage both hemispheres, which wastes attentional resource that could otherwise be utilised for various other aspects of ongoing processing.