Tapani Korhonen

Cortical and subcortical visual event-related potentials to oddball stimuli in rabbits.

&NA; Event‐related potentials (ERPs) to changes in the visual environment were recorded in rabbits. In the oddball condition, infrequently presented (deviant) stimuli occurred in a series of frequently presented (standard) stimuli. In the deviant‐alone condition, standards were omitted. ERPs to oddball‐deviants differed from those to standards in all recording sites (cerebellar cortex, visual cortex, dentate gyrus). No corresponding differences were found between ERPs to deviants in the oddball condition and those in the deviant‐alone condition. However, because ERPs to deviants in the deviant‐alone condition and those to standards did not differ either, ERPs to stimulus changes in the oddball condition seemed to be dependent on the presence of standards, thus representing an analogue to mismatch negativity (MMN) in humans.

Auditory cortical event-related potentials to pitch deviances in rats

We recorded epidural event-related potentials (ERPs) from the auditory cortex in anesthetized rats when pitch-deviant tones were presented in a homogeneous series of standard tones (oddball condition). Additionally, deviant tones were presented without standard tones (deviant-alone condition). ERPs to deviant tones in the oddball condition differed significantly from ERPs to standard tones at the latency range of 63-243 ms. On the other hand, ERPs to deviant tones in the deviant-alone condition did not differ from ERPs to standard tones until 196 ms from stimulus onset. The results suggest that oddball stimuli can be neurophysiologically discriminated in anesthetized rats. Furthermore, as the difference between ERPs to deviant tones and those to standard tones at the 63-196 ms latency range could be detected only when standard tones precede deviant tones it shows concordance with mismatch negativity in humans.

The human brain processes visual changes that are not cued by attended auditory stimulation.

Event-related potentials (ERPs) to visual stimuli were recorded from the scalp of eight adult humans performing a task in which they counted vowels from a heard story. In the oddball condition, a repeated (standard) light bar of 50 ms in duration was rarely (P = 0.1) replaced by a (deviant) one differing in orientation from the standard. In the control condition, standards were simply omitted from the series and only (alone-) deviants retained. In both conditions, visual stimuli were asynchronous with auditory-task-relevant stimuli. ERPs to deviants significantly differed in amplitude from those to standards in the midline electrodes centrally, parietally and occipitally at 160-200 ms from stimulus onset. Occipitally, such a difference was absent between ERPs to alone-deviants and those to standards. The occipital differential ERPs to deviants, which thus could be found only when standards were present in the series, are discussed in the context of the mismatch negativity (MMN).

Hippocampal theta (3-8 Hz) activity during classical eyeblink conditioning in rabbits

In 1978, Berry and Thompson showed that the amount of theta (3-8Hz) activity in the spontaneous hippocampal EEG predicted learning rate in subsequent eyeblink conditioning in rabbits. More recently, the absence of theta activity during the training trial has been shown to have a detrimental effect on learning rate. Here, we aimed to further explore the relationship between theta activity and classical eyeblink conditioning by determining how the relative power of hippocampal theta activity [theta/(theta+delta) ratio] changes during both unpaired control and paired training phases. We found that animals with a higher hippocampal theta ratio immediately before conditioning learned faster and also that in these animals the theta ratio was higher throughout both experimental phases. In fact, while the hippocampal theta ratio remained stable in the fast learners as a function of training, it decreased in the slow learners already during unpaired training. In addition, the presence of hippocampal theta activity enhanced the hippocampal model of the conditioned response (CR) and seemed to be beneficial for CR performance in terms of peak latency during conditioning, but did not have any effect when the animals showed asymptotic learning. Together with earlier findings, these results imply that the behavioral state in which hippocampal theta activity is absent is detrimental for learning, and that the behavioral state in which hippocampal theta activity dominates is beneficial for learning, at least before a well-learned state is achieved.

Reflex facilitation during eyeblink conditioning and subsequent interpositus nucleus inactivation in the rabbit (Oryctolagus cuniculus).

In eyeblink conditioning in the rabbit (Oryctolagus cuniculus), not only is a conditioned response (CR) acquired, but also the original reflex is modified as a function of training. In Experiment 1, by comparing unconditioned responses in unpaired and paired groups, 3 types of reflex facilitation were distinguished. One type was linked to exposure to the unconditioned stimuli (USs) and/or experimental setting. The 2nd type was related to the formation of the memory trace for conditioned eyeblink. The 3rd type was linked to the conditioned stimulus immediately preceding the US in the paired group. In Experiment 2, reversible inactivation of the interpositus nucleus (IPN) abolished the CR and reduced the CR-related reflex facilitation, indicating that the latter depends on the plasticity of the IPN.

ERPs to pitch changes: a result of reduced responses to standard tones in rabbits.

Event-Related potentials (ERPs) were recorded in rabbits when pitch deviant tones occurred in a series of standard tones (oddball situation). In control recordings, the deviant tones were presented without the standard tones (deviant-alone situation). In the oddball situation, significant difference ERPs (deviant ERPs-standard ERPs) could be found in the hippocampal and cerebellar recordings but not in the visual cortex. All the ERPs to the deviant stimuli observed in the oddball situation were also present in the deviant-alone situation. The difference ERPs were therefore based on reduced responses to the standards. The results are discussed in the context of a mismatch negativity (MMN) in humans.

A method for rapid implantation of multielectrode systems

The implantation of several chronic electrodes at the same time was needed for the studies in which both evoked slow potential and multiple unit neural activity of different brain sites were recorded during classical conditioning in freely moving cats. The recording electrodes were made of 25-75 microns insulated stainless steel or nichrome wires which were inserted into a thin hypodermic needle of a fixed, calibrated length. The fast implantation system described here is based on the use of a stereotaxic frame as a stand for a drill and as a support for an electrode holder with which the electrodes were implanted through precision holes drilled directly according to the coordinates of the brain atlas. The connecting pins of each electrode were inserted to a connector during implantation. The described implantation system allows short distances (0.5 mm) between electrode shafts and free determination of the depth of each electrode. Because fixing of an individual electrode with dental cement to the skull is not needed during the initial phase and the precalibrated electrodes need no adjustments during implantation, considerable time savings can be achieved.

Interpositus nucleus inactivation reduces unconditioned response amplitude after paired but not explicitly unpaired treatment in rabbit eyeblink conditioning.

The amplitude of unconditioned responses (URs) in unconditioned stimulus (US) alone presentations were measured in six rabbits during explicitly unpaired and classical conditioning treatments. After both phases of the experiment, the interpositus nucleus (IPN) was reversibly inactivated by a cold probe. URs after unpaired treatment were unaffected by inactivation but after acquisition of a robust level of conditioned responses (CRs), URs in US-alone test trials were reduced in amplitude compared with URs immediately before and after inactivation. The results suggest that the IPN has a role in CR-related reflex modification.

Somatosensory event-related potentials in the rabbit cerebral and cerebellar cortices: a correspondence with mismatch responses in humans

Somatosensory event-related potentials (ERPs) were recorded from the cerebellar (CerCx), somatosensory (SomCx) and visual (VCx) cortices in rabbits in two stimulus conditions. In the oddball condition, airpuffs to two different locations in the rabbits muzzle corresponded to infrequently presented deviant stimuli (oddball-deviants) interspersed with frequently presented standard stimuli. In the deviant-alone condition, deviants (alone-deviants) were presented without standards. ERPs to oddball-deviants differed significantly from those to standards in CerCx and SomCx, but not in VCx. Furthermore, some of these differences were not found between ERPs to alone-deviants and those to standards. Thus, ERPs to oddball-deviants contributing these differences were dependent on the presence of preceding standards. The results are in line with the elicitation of mismatch negativity-like ERPs to somatosensory changes in humans.

Hippocampal evoked potentials to pitch deviances in an auditory oddball situation in the rabbit: no human mismatch-like dependence on standard stimuli.

Hippocampal auditory evoked potentials (AEP) were recorded in 10 rabbits when pitch deviant tones occurred in a series of standard tones (oddball situation). In control recordings, deviant tones were presented without intervening standard tones (deviant-alone situation). All AEP deflections observed in the oddball situation were found also in the deviant-alone situation. Thus, it appeared that none of the AEP deflections to deviant tones in the oddball situation was specific to a memory trace of preceding standard tones. This observation was in contradiction to such a specificity of the mismatch negativity (MMN) found in humans. Instead, a connection to a neuronal orienting reaction interpretation was shown to be apparent. A need for additional control procedures revealing a contribution of neural orienting responses in animal MMN experiments is discussed.