Temporal onset-order discrimination through the tactual sense: effects of frequency and site of stimulation

This research extends the study of temporal resolution of the tactual sensory system through measurements of temporal-onset order discrimination for continuous tonal signals addressing (a) the effects of frequency separation of the two stimuli whose onset orders are to be discriminated and (b) the effects of redundant coding of frequency and site of stimulation on performance. Sinusoidal signals were presented either at two separate digits (thumb and index finger of the left hand) or at a single site of stimulation (left index finger) using a multifinger tactual stimulation system. Measurements were obtained using a one-interval two-alternative forced choice procedure in which each interval consisted of the random-order presentation of two different stimuli with roving values of amplitude and duration. Thresholds were estimated from psychometric functions of d' as a function of stimulus-onset asynchrony (SOA). On average, temporal onset-order thresholds were larger for one-finger conditions (mean SOA of 74.8 ms) than for two-finger conditions (mean SOA of 48.5 ms) and decreased as frequency separation increased, particularly for single-site presentation. Redundant coding of frequency and site of stimulation resulted in higher resolution by a factor of 1.5 compared to frequency alone and by a factor of 1.2 compared to site alone.     

Integration of auditory and vibrotactile stimuli: effects of frequency

Perceptual integration of vibrotactile and auditory sinusoidal tone pulses was studied in detection experiments as a function of stimulation frequency. Vibrotactile stimuli were delivered through a single channel vibrator to the left middle fingertip. Auditory stimuli were presented diotically through headphones in a background of 50 dB sound pressure level broadband noise. Detection performance for combined auditory-tactile presentations was measured using stimulus levels that yielded 63% to 77% correct unimodal performance. In Experiment 1, the vibrotactile stimulus was 250 Hz and the auditory stimulus varied between 125 and 2000 Hz. In Experiment 2, the auditory stimulus was 250 Hz and the tactile stimulus varied between 50 and 400 Hz. In Experiment 3, the auditory and tactile stimuli were always equal in frequency and ranged from 50 to 400 Hz. The highest rates of detection for the combined-modality stimulus were obtained when stimulating frequencies in the two modalities were equal or closely spaced (and within the Pacinian range). Combined-modality detection for closely spaced frequencies was generally consistent with an algebraic sum model of perceptual integration; wider-frequency spacings were generally better fit by a Pythagorean sum model. Thus, perceptual integration of auditory and tactile stimuli at near-threshold levels appears to depend both on absolute frequency and relative frequency of stimulation within each modality.     

Halide-induced supramolecular ligand rearrangement

A novel reaction involving the halide-induced rearrangement of ligands within supramolecular Rh(I) complexes containing hemilabile ligands is presented. Three analogous bis- and trishemilabile ligands have been synthesized to construct bi- and trimetallic Rh(I) macrocyclic complexes. An intentionally added halide source results in the formal rotation of only one hemilabile ligand along the axis that is perpendicular to the plane defined by the aryl backbone of the hemilabile ligands. X-ray structures, as determined by X-ray crystallography, of key intermediates and products are presented.