AVAYA

(a). For acceleration steps

100 - e

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Note: SD = S tandard deviation

Fig. 3-4: Psychometric functions (continuous line for normal subjects and dashed line for LDs) fitted to the average proportions of correct responses (circles for normal subjects and stars for LDs) vs. acceleration levels, for 8 normal subjects and 5 LDs. Bars at the bottom of the graph show the percentages of stimuli dispensed around one acceleration level (black bars for normal subjects and white bars for LDs). Interval bars connected to the data points show +1 SD for the proportions of correct responses.

Note 1: as the adaptive procedure delivered tlie maximiun number o f stimuli for every other acceleration level, only results corresponding to these levels are plotted. The percentage o f stimuli delivered at one acceleration level a was calculated as: percentage delivered at level a + 0.5 x (percentage delivered at the level below a + percentage delivered at the level above a).

Note 2: the mean threshold for a group of subjects can only be estimated on this figure, since the data points were calculated from the logistic functions associated with each subject; this calculation enabled us to take all subjects into account for each level of accelerations.

The choice of the imderlying psychometric function was validated by the strong correlation between predicted and observed proportions of correct responses, mean coefficients of correlation of 0.92 (range; 0.73 - 0.99) for normal subjects and 0.91 (range: 0.8 - 0.99) for LDs. The mean thresholds for 67% correct detections of direction were 4.84 cm/s^ (range: 2.88 - 6.28 cm/s^) in normal subjects and 6.85 cm/s^ in LDs; 4.85 and 6.59 cm/s^ (idiopathic), 5.63 and 12.30 cm/s" (meningitic), 4.97 cm/s" (neurectomy). The patient with 12.3 cm/s^ took part in other similar

Chapter Three - Perception of linear motion

experiments, and on later retesting, lowered his threshold to 6.2 cm/s^ showing an ability to learn how to detect motion direction for low acceleration levels (new mean threshold for LDs of 5.65 cm/s^, new range; 4.85 - 6.6 cm/s^). He reported concentrating on perception of pressure on his body. The purpose of employing an adaptive procedure was to generate a maximum number of stimuli at liminal intensities. As can be observed in Fig. 3-4, the majority of the stimuli, 70% for normal subjects and 67% for LDs, was dispensed at acceleration levels between 3.9 cm/s^ and 6.5 cm/s^ therefore bracketing threshold values. The minimum and maximum acceleration steps were delivered at 2 and 11 cm/s^.

(b). For linear and parabolic accelerations

Normal subjects

Proportions o f correct responses

The minimum of 7 correct responses necessary to determine thresholds with Analysis 1 was not obtained for one subject during SlowR and for four others during Par. Taking into account all subjects including those presenting less than 7 correct responses, the average percentages of correct responses at deceleration onsets were 76.6% (range: 50-90%) for SlowR, 88.9% (70-100%) for FastR and 72.2% (60-100%) for Par. Similar mean values and identical ranges were obtained when only the first responses (i.e. ignoring any late corrective indication) were considered: 74% for SlowR, 86% for FastR and 74% for Par. These percentages indicated that detecting motion direction was more difficult during Par and SlowR than during FastR.

Taking these percentages into consideration, both techniques of analysis were slightly adjusted to improve the comparison of thresholds associated to each stimulus. Analysis 1 was divided into three conq)arisons between two stimuli only, excluding the results from the subjects for whom thresholds could not be determined for at least one of the two waveforms. For Analysis 2, threshold values for FastR corresponding to a proportion of correct response for the whole group of subjects equal to 74% (percentages obtained for SlowR and FastR) were calculated: to do so, for the subjects whose proportion of correct detection for FastR was higher than for SlowR, the responses with the longer latencies were considered as undetected direction (therefore, the corresponding latency was ignored), until the percentage of correct detection was equal to the percentage for SlowR and at least greater than 60%.

Thresholds

Table 3-2 shows that thresholds obtained with both analyses were similar. However, Analysis 2 had the advantages of including data from all subjects and of being based on the same numbers of correct responses, making the conq)arisons between stimuli easier. Therefore, the following paragraphs will focus on the results obtained with this second technique.

Chapter Three - Perception of linear motion

FastR SlowR Par

cm/s^ % cm/s^ % cm/s^ % ANALYSIS 1 FastR / SlowR (8) 25.9 (13.6-43.6) 89 14.4 (9.9-20.9) 80 FastR / Par (5) 22.6(13.6-32.1) 92 19.1 (14.2-29.9) 84 Par / SlowR (4) 12.2 (9.9-16.6) 82 16.4 (14.2-20.3) 87 ANALYSIS 2 (9) 19.2 (10.4-35.3) 74 12.1 (7.3-20.4) 74 16.7 (10.5-25.0) 74

Table 3-2: Acceleration thresholds (mean and range), proportions o f correct responses indicated at deceleration onset (Analysis 1) or proportions o f correct initial responses (Analysis 2). The numbers in brackets in the first column are the numbers o f subjects taken into account.

Note: for Analysis 2, the acceleration threshold associated to FastR was 23.1 cm/s^ for a percentage o f correct responses o f 86%.

It appears that acceleration levels at which motion direction was detected were dependent on stimulus profile, which is most evident when the two linear accelerations are considered. This inq)lies that acceleration was not the sole parameter influencing the process of motion perception.

The average response latencies were 2.9 s (range: 1.4-5.1) for SlowR, 1.6 s (0.8-3.0) for FastR and 2.7 s (1.8-3.8) for Par, values already corrected with the mean reaction time of 490 ms (range: 302-760 ms). Thus, subjects indicated their motion direction earlier during FastR than during SlowR and Par, but the highest mean acceleration thresholds were obtained for FastR. This suggests that the other parameter which ought to be taken into account to explain motion perception is velocity; the tenq)oral integration of acceleration. Incidentally, as the stimulus durations were always more than twice the mean latencies for signalling motion direction, it can be concluded that the chair was accelerated for a sufficient time to permit motion perception and therefore threshold determination.

Patients

Two of the 3 LDs had results within the normal subjects’ ranges for all stimuli (Analysis 2). PI had thresholds close to the mean values of normal subjects: 13.2 cm/s^ for SlowR (proportion of correct responses of 60%), 21.4 cm/s^ for FastR (80%) and 18.4 cm/s^ for Par (80%). This subject was accustomed to the test and reported that he concentrated on the pressure exerted on his body, by the clangs, to detect his motion direction. P2 had thresholds in the lower part of the normal range for SlowR (20 cm/s^, 90%), FastR (30.6 cm/s^, 70%) and Par (19.1 cm/s^, 70%) and as normal subjects, was unaware of the way he detected motion. P4, who had a normal threshold of 4.97 cm/s^ for acceleration steps, only indicated between 40 and 50% of correct responses for linear and parabolic stimuli, which shows her inability in detecting motion direction for displacements with low acceleration gradients.

Chapter Three - Perception of linear motion