Spin Fine Structure Reveals Biexciton Geometry in an Organic Semiconductor.

In organic semiconductors, biexcitons are key intermediates in carrier multiplication and exciton annihilation. Their local geometry governs their electronic properties and yet has been challenging to determine. Here, we access the structure of the recently discovered S=2 quintet biexciton state in an organic semiconductor using broadband optically detected magnetic resonance. We correlate the experimentally extracted spin structure with the molecular crystal geometry to identify the specific molecular pairings on which biexciton states reside.

Site-selective measurement of coupled spin pairs in an organic semiconductor.

From organic electronics to biological systems, understanding the role of intermolecular interactions between spin pairs is a key challenge. Here we show how such pairs can be selectively addressed with combined spin and optical sensitivity. We demonstrate this for bound pairs of spin-triplet excitations formed by singlet fission, with direct applicability across a wide range of synthetic and biological systems. We show that the site sensitivity of exchange coupling allows distinct triplet pairs to be resonantly addressed at different magnetic fields, tuning them between optically bright singlet ([Formula: see text]) and dark triplet quintet ([Formula: see text]) configurations: This induces narrow holes in a broad optical emission spectrum, uncovering exchange-specific luminescence. Using fields up to 60 T, we identify three distinct triplet-pair sites, with exchange couplings varying over an order of magnitude (0.3-5 meV), each with its own luminescence spectrum, coexisting in a single material. Our results reveal how site selectivity can be achieved for organic spin pairs in a broad range of systems.

Sigma-optokinetic nystagmus in squirrel monkeys elicited by stationary stripe patterns illuminated by regular and random-interval flash sequences.

Eye position and angular velocity were measured in squirrel monkeys (Saimiri sciureus) by means of the electromagnetic scleral search-coil technique. Horizontal sigma-optokinetic nystagmus (sigma-OKN) was elicited by a stationary, stroboscopically illuminated, periodic, vertical-stripe pattern lining a vertical cylinder. The relationship between the mean slow-phase eye angular velocity, Ve, of sigma-OKN and the product of pattern period, Ps, and flash frequency, f(s), was determined. When Ve approximated k x Ps x f(s) (deg x s(-1)) and k was an integer > or = 1, the sigma-paradigm was fulfilled. Sigma-OKN could be evoked in different "modes", whereby k approximated 1, 2,...n. The sigma-OKN properties of squirrel monkeys were similar to those measured for sigma-OKN in the "stare" mode in man, with the exception of a long-lasting optokinetic afternystagmus (sigma-OKAN) appearing in the monkey. A considerable amount of temporal variability in flash sequence intervals ("temporal noise"), causing retinal error signals that interfered with the sigma-paradigm, was accepted by the visuo-motor system without interruption of sigma-OKN. This observation was explained by the operation of a short memory device for perception of visual motion. The internal gain, g(i), which relates the retinal "error" displacement velocity, Vr, and Ve depended, in turn, on Vr according to a function resembling the known relationship between neuronal activity of NOT (nucleus of the optic tract) nerve cells and Vr. This observation may be taken as direct proof that sigma-OKN can be explained by a centrally preprogrammed relationship between the retinal velocity, Vr, and the OKN slow-phase eye velocity, Ve. It is stipulated that the sum of Vr and efference copy signals generated in cortical or subcortical gaze centers is the essential component controlling the perceived velocity of the sigma-movement, whereby a short-term integrator plays a role for squirrel monkey sigma-OKN. When the flash frequency, f(s), was modulated periodically according to a sinewave or "triangular" function at a rate below 0.5 cycles x s(-1), Ve was found to respond with a corresponding modulation, provided the modulation amplitude did not exceed 50% of the mean flash rate. When the latter occurred, nonlinear responses could be observed. A similar response was found when the speed of "real" optokinetic stimuli was varied sinusoidally. Under these experimental conditions, however, the amplitude of the Ve variation yielded up to 1.0 approximately linear responses.

An automated and modified technique for testing the retinal function (Arden test) by use of the electro-oculogram (EOG) for clinical and research use.

The electro-oculogram (EOG) can be recorded by use of skin electrodes positioned near the eye in the horizontal or vertical plane and is dependent on the position of the eye relative to the electrodes and the size of the corneo-retinal potential (CRP). By use of the functional relation between the recorded potential and the eye position - CRP is considered to be constant - the EOG is the eye movement recording method most often used. Changes in CRP dependent on luminance or metabolic influences can be measured, on the other hand, by means of the EOG when eye movements of constant amplitudes are performed. The EOG in this paper is used in the latter sense. The widely used clinical test of rentinal function based on the electro-oculogram firstly described by Arden (Arden index) has been modified to achieve more precise and reliable results by including a gradual dark adaptation procedure and a special data analysis to detect fixation periods. Using modern techniques, the complete procedure has been automated. The efficiency of the procedure and the device is demonstrated in various ways.

An algorithm separating saccadic from nonsaccadic eye movements automatically by use of the acceleration signal.

An algorithm is described to discriminate automatically between saccades and slow eye movements. Sampled data of the eye position have been used to calculate the momentary acceleration of the eye. The higher acceleration values of the saccadic eye movements as opposed to the slow compensatory or pursuit eye movements served to differentiate between the two. The method is demonstrated by search-coil data in squirrel monkeys.

A study of the encoder properties of muscle-spindle primary afferent fibers by a random noise disturbance of the steady stretch response.

Muscle spindle stretch responses (cat gastrocnemius muscles) were studied when both steady stretch and small near-threshold random stretch determined the Ia impulse sequence. Statistical properties of the inter-impulse-intervals gave some insight into the Ia encoder mechanism. Superimposed random stretch of mean velocities sigma vel below 5 mm s-1 did not change the mean discharge rate, but the width of the Ia interspike interval distribution was clearly increased. Raising the stretch velocity further (sigma vel greater than 5 mm s-1) led to an additional increase in the distribution width, finally reaching values of 0.6 for the coefficient of variation. The shapes of the impulse interval histograms changed from symmetrical to positively skewed ones. The 1st order serial correlation coefficient of the interval sequence shifted to slightly more negative values with increasing sigma vel; on the average, the r1,2-value varied between zero and -0.2. The data were discussed in relation to current ideas on the mechanism of impulse initiation in the Ia terminal ending. They provide evidence that a combination of multiple encoder sites located in the myelinated terminal branches and a separate pathway for large static and small-amplitude dynamic stretch is not very likely. A model is proposed as to how the whole tree of myelinated axons functions as a single encoder site.

Observations on phase-locking within the response of primary muscle spindle afferents to pseudo-random stretch.

In order to uncover encoder properties of primary muscle spindle afferent fibers, time coupling (phase-locking) of action potentials on cyclic muscle stretch was studied by means of pseudo-random noise. In cats Ia action potentials were recorded from dorsal root filaments and the gastrocnemius muscles of one hind leg were stretched. The stimulus time course was a determined sequence of randomly varying muscle length which could be applied repeatedly (sequence duration 0.6 or 20 s). The noise amplitude sigma (standard deviation of displacements) was varied between 5 and 300 micron, the upper cut-off frequency of noise fc was varied between 20 and 100 Hz. The responses to the consecutive pseudo-random noise cycles were displayed as raster diagrams and cycle histograms. Phase-locking characterized the responses at all noise amplitudes outside the near threshold range (sigma greater than 10 micron). The higher sigma and fc, the stronger was the phase-locking of impulses on the stretch. When sigma and fc were selected to achieve high mean stretch velocities of about 500 mm/s, phase-locking was as precise as 0.15 ms, measured as the variability of spike occurrences with respect to stretch. The rasters obtained with low noise amplitudes (less than 40 micron) showed a loose phase-locking and this gave insight into underlying mechanisms: The elicitation of action potentials caused by dynamic stretch can be prevented by a post-spike depression of excitability. This disfacilitation was very effective in counteracting weak stretch components within the random sequence and less effective or even missing when relatively strong stretch components could force the spike elicitation.(ABSTRACT TRUNCATED AT 250 WORDS)

Superimposing noise linearizes the responses of primary muscle spindle afferents to sinusoidal muscle stretch.

Experiments were conducted in anaesthetized and spinalized cats to measure the extent to which the non-linear response of Ia afferent fibers to sinusoidal muscle stretch as expressed by the peristimulus-time-histograms, PSTHs, can be transformed into a linear one by means of the superposition of random stretch ("mechanical noise"). The gastrocnemius muscles of one hind leg were stretched and the response to sinewave muscle stretch (amplitudes between 0.01 and 4.0 mm, frequencies between 0.1 and 20 Hz) were investigated while band-limited mechanical noise was superimposed on the sinewave stretch. The random stretch upper cut-off frequency was varied between 60 and 300 Hz; the displacements were normally distributed. The noise amplitude sigma, i.e. the standard deviation of the displacement distributions, was varied systematically between 0.002 and 0.4 mm. Mechanical noise was very effective in raising the mean discharge rate. Added to the sinusoidal stretch it prevented the cessation of firing during the release phase of the stretch cycle, or at least reduced the duration of discharge pauses, i.e., a linearization occurred. In general, the larger the noise amplitude, the more the amplitude of the fundamental harmonic component was attenuated and the phase lead reduced. Apart from this rule the particular combination of superimposing small noise (sigma less than 0.02 mm) on small sinewave stretch (A less than 0.02 mm) could enhance the depth of sinusoidal modulation of cycle histograms (compared with responses to pure sinusoids). Linearizing the sinewave response by additional noise allowed the estimation of frequency response characteristics in the otherwise non-linear range of amplitudes (sinewave amplitude 0.5-1.0 mm).(ABSTRACT TRUNCATED AT 250 WORDS)

Afterimage movement during saccades in the dark.

The spatial values of retinal coordinates are "recalibrated" to the "egocentric" coordinates during and after a saccade within a fraction of a second. We measured the time constant of this retinal coordinate transformation by means of an afterimage technique: our ten subjects performed "auditory" horizontal saccades in total darkness (0.2-4.5 saccades/sec). At a saccade frequency below 1 saccade/sec, the subjects observed saccadic displacement of the foveal afterimage, but the afterimage seemed to arrive at its final position more slowly than the center of gaze (state 1). At saccade frequencies above 1.5 saccades/sec, the perceived amplitude of afterimage displacement decreased with increased saccade frequency (state 2). Above 2 saccades/sec all subjects perceived two stationary afterimages simultaneously at the saccadic end-position (state 3). A further increase in saccade frequency reduced the distance between the two afterimages till only one stationary afterimage was seen in a mid-position between the two auditory targets at a saccade frequency above 3.2-3.5 saccades/sec (state 4). Saccade amplitude remained constant within the frequency range between 0.2 and 4.5 saccades/sec. A one-step or two-step linear model was applied to simulate the experimental data, indicating that the spatial coordinates shift more slowly than the saccadic eye movements.

The response of primary muscle spindle endings to random muscle stretch: a quantitative analysis.

In cats the stretch responses of deafferented Ia afferent fibers from gastrocnemius muscle spindles were measured. The response properties were derived by correlating the action potentials to random mechanical stimuli applied to the gastrocnemius muscle. The muscle stretch consisted of band-limited, normally distributed random changes in muscle length. The upper cut-off frequency of the noise stimuli fc (i.e. the -3 dB point of the amplitude spectrum) was varied systematically between 1.5 and 600 Hz and the amplitude of the noise stimuli sigma (i.e. the standard deviation of the distribution of muscle displacements) between 1.5 and 1200 microns. The neuronal responses were analysed by averaging the stimulus signals centered on the Ia action potentials (the "peri-spike average", PSA). The cross-spectrum between stimulus and neuronal response was determined by computing the Fourier expansion of the PSA. Immediately before the action potentials the PSA decreased below mean muscle length and increased thereafter rapidly. As a rule, the action potentials were elicited close to the point of maximum stretch velocity. The cross-spectra revealed a monotonic increase in gain with stimulus frequency, corresponding to a power function with an average exponent of 0.86. The PSA-shape obtained at stimulus cut-off frequencies between 1.5 and 260 Hz remained fairly independent of fc when the abscissa was normalized with 1/fc. In the cross-spectra a maximum gain was always reached near the actual fc-value for upper cut-off frequencies up to 260 Hz. Above 260 Hz maximum gain remained independently of fc at about 260 Hz. The peak-to-peak displacement A of the PSA increased linearly with the logarithm of fc. The shapes of the PSAs and the cross-spectra did not depend on the noise amplitude sigma in the range above 20 microns. A was linearly related to sigma (about 1.6 to 2.4 . sigma for sigma greater than 10 microns and 75 Hz less than fc less than 450 Hz). A unifying mathematical description of the responses, valid for a wide range of stretch parameters, was obtained. We demonstrated that the application of one short run of random muscle stretch of an intermediate noise amplitude and medium cut-off frequency provides substantial information on the spindle stretch response and is recommended for investigations in which stable recordings cannot be obtained for a sufficient length of time.

Effect of hexachlorophene intoxication on learning in rats.

Neurotoxicity in the form of hindlimb paralysis is known to be associated with brain lesions characterized by vacuolation of the white matter following hexachlorophene (HCP) intoxication; these paralytic and histopathological effects are reversible with time after discontinuance of HCP exposure. The rates of acquiring a bar-pressing escape and avoidance behavior were measured in female rats after recovery from paralysis following daily oral dosing with HCP (25 mg/kg). Learning deficits were seen as a delay in reaching a 90% escape performance level, as an increase in the number of sessions required to reach a 50% avoidance criterion, as an increase in training time needed to transfer from escape to avoidance responding, and by an increase in the number of sessions needed to reach a maximum level of avoidance behavior over a 35-session period. Peak responding eventually reached a level comparable to controls only after prolonged periods. Brain lesions seen 2-3 months after HCP intoxiciation correlated with the lag in learning ability but not with the maximum avoidance acquisition before sacrifice. These results indicate a long-lasting behavioral deficiency following neurotoxicity which is slowly reversible.