Evidence of impaired naming in patients with hippocampal amnesia

The hippocampus supports precise memory for public events regardless of their remoteness

The necessity of the human hippocampus and surrounding medial temporal lobe structures to semantic memory remains contentious. Impaired semantic memory following hippocampal lesions could arise either due to partially intertwined episodic memories and/or retrograde/anterograde effects. In this study, we tested amnesic individuals with lesions in hippocampus and surrounding medial temporal lobe (n = 7) and age-matched controls (n = 14) on their ability to precisely recall the dates of famous public events that occurred either before (i.e., pre-lifetime) or after participants' birth date (lifetime). We show that deficits in dating precision are greatest for recent lifetime events, consistent with the notion that recent event memory may be particularly intertwined with episodic memory. At the same time, individuals with medial temporal lobe lesions showed more subtle impairments in their ability to date pre-birth and remote lifetime events precisely. Together, these findings suggest that the hippocampus and surrounding medial temporal lobe structures are important for representational precision of semantic memories regardless of their remoteness.

Maintenance of Procedural Motor Memory across Brief Rest Periods Requires the Hippocampus

Research on the role of the hippocampus in memory acquisition has generally focused on active learning. But to understand memory, it is at least as important to understand processes that happen offline, during both wake and sleep. In a study of patients with amnesia, we previously demonstrated that although a functional hippocampus is not necessary for the acquisition of procedural motor memory during training session, it is required for its offline consolidation during sleep. Here, we investigated whether an intact hippocampus is also required for the offline consolidation of procedural motor memory while awake. Patients with amnesia due to hippocampal damage (n = 4, all male) and demographically matched controls (n = 10, 8 males) trained on the finger tapping motor sequence task. Learning was measured as gains in typing speed and was divided into online (during task execution) and offline (during interleaved 30 s breaks) components. Amnesic patients and controls showed comparable total learning, but differed in the pattern of performance improvement. Unlike younger adults, who gain speed across breaks, both groups gained speed only while typing. Only controls retained these gains over the breaks; amnesic patients slowed down and compensated for these losses during subsequent typing. In summary, unlike their peers, whose motor performance remained stable across brief breaks in typing, amnesic patients showed evidence of impaired access to motor procedural memory. We conclude that in addition to being necessary for the offline consolidation of motor memories during sleep, the hippocampus maintains access to motor memory across brief offline periods during wake.

Evidence of Impaired Remote Experience-near Semantic Memory in Medial Temporal Lobe Amnesia

Neuropsychological research suggests that "experience-near" semantic memory, meaning knowledge attached to a spatiotemporal or event context, is commonly impaired in individuals who have medial temporal lobe amnesia. It is not known if this impairment extends to remotely acquired experience-near knowledge, which is a question relevant to understanding hippocampal/medial temporal lobe functioning. In the present study, we administered a novel semantic memory task designed to target knowledge associated with remote, "dormant" concepts, in addition to knowledge associated with active concepts, to four individuals with medial temporal lobe amnesia and eight matched controls. We found that the individuals with medial temporal lobe amnesia generated significantly fewer experience-near semantic memories for both remote concepts and active concepts. In comparison, the generation of abstract or "experience-far" knowledge was largely spared in the individuals with medial temporal lobe amnesia, regardless of whether the targets for retrieval were remote or active concepts. We interpret these findings as evidence that the medial temporal lobes may have a sustained role in the retrieval of semantic memories associated with spatiotemporal and event contexts, which are cognitive features often ascribed to episodic memory. These results align with recent theoretical models proposing that the hippocampus/medial temporal lobes support cognitive processes that are involved in, but not exclusive to, episodic memory.

The growing gap: A study of sleep, encoding, and consolidation of new words in chronic traumatic brain injury

Word learning is an iterative and dynamic process supported by multiple neural and cognitive systems. Converging evidence from behavioral, cellular, and systems neuroscience highlights sleep as an important support for memory and word learning over time. In many lab-based word learning experiments, participants encode and subsequently retrieve newly learned words in a single session. These designs are inadequate to capture the full dynamic word learning process, making them less ecologically valid. Single timepoint studies also limit investigation of the role of behavioral and lifestyle factors, like sleep, in supporting word learning over time. Adults with a history of traumatic brain injury (TBI), who commonly exhibit deficits in the memory systems that support word learning and report concomitant sleep disturbance, provide a unique opportunity to examine the link between memory, sleep, and word learning. Here we examined word learning over time and the influence of sleep on short- and long-term word recall in 50 adults with chronic moderate-severe TBI and 50 demographically matched neurotypical peers. We used a randomized within-participant crossover design to assess immediate encoding of new words and the consolidation of those words over time across intervals that did or did not involve sleep. Participants completed this study over the course of two weeks in their own homes to capture the iterative, dynamic process of real-world word learning. We also measured sleep in free living conditions using actigraphy throughout the experiment. Participants with TBI exhibited a word learning deficit that began at encoding and persisted across time. Critically, this deficit grew over the course of the week. The performance gap between groups was larger at the 1-week post-test than the immediate post-test, suggesting deficits in both encoding and consolidation of new words in individuals with TBI. Participants with and without TBI remembered more words when they slept after learning. Ecologically valid research designs that examine the relationship between memory, sleep, and word learning over time promise to advance mechanistic accounts of word learning and improve the long-term retention of new words in individuals with and without brain injury.

Word Learning as a Window to Memory and Rehabilitation Outcomes in Traumatic Brain Injury

PURPOSE

The purpose of this viewpoint is to advocate for increased study of word learning abilities and word learning interventions in traumatic brain injury (TBI).

METHOD

We describe the word learning process and the unique opportunities afforded by studying each component and stage. Building on discussions at the 2022 International Cognitive-Communication Disorders Conference, we describe how word learning may underlie a variety of functional outcomes after TBI, making it a promising target for rehabilitation. Finally, we discuss principles that may guide study in this critical area to advance outcomes after TBI for children and adults.

RESULTS

Word learning is a dynamic and iterative process taking place over time and in multiple stages. Thus, studying word learning affords the opportunity to parse the relative contributions of multiple memory systems to different phases and components of the process. However, single-timepoint designs are insufficient to capture the full word learning process, which occurs over time and across contexts. Word learning also presents an opportunity to assess the contributions of behavioral and lifestyle factors (e.g., sleep and exercise) to different memory phases. Understanding these interactions could drive clinical interventions aimed at improving memory through manipulable external behaviors.

CONCLUSIONS

Word learning is key to success in functional spheres across the life span. The importance of words to daily life remains after TBI, even as the memory systems that support word learning are disrupted. The empirical study of word learning and rehabilitation of word learning deficits in TBI presents a promising new direction in understanding the breadth of neurogenic cognitive-communication disorders and an opportunity to explore a potential driver of functional outcome and impactful rehabilitation target.

Risk factors and screening for neurocognitive impacts of therapy

Long-term survivors of pediatric hematologic malignancies are at elevated risk for neurocognitive impairment. Such impairment manifests in different ways at different times during survivorship, with deficits in processing speed, attention, and memory often appearing before deficits in executive function, intelligence, and academics. Survivors exposed to therapies that directly target the central nervous system (CNS), as is the case in acute lymphoblastic leukemia, may demonstrate subtle deficits during frontline therapy, and these deficits may grow and evolve over time. Survivors who do not receive CNS-directed therapies (eg, Hodgkin lymphoma) are also at elevated risk for neurocognitive impairment, although the influence on brain function is indirect through cancer therapy impact on systemic organ function vital to brain health (eg, cardiopulmonary morbidity). Over the course of the survivor's life span, the presence and impact of neurocognitive deficits will be determined by a complex interaction between premorbid development and environment, cancer therapy and clinical care, and posttreatment recovery and health. The timing and type of these treatment and health events will dictate the approach to screening and monitoring for neurocognitive impairment.

The importance of basal-temporal white matter to pre- and post-surgical naming ability in temporal lobe epilepsy

OBJECTIVE

Emerging research highlights the importance of basal-temporal cortex, centered on the fusiform gyrus, to both pre-surgical naming ability and post-surgical naming outcomes in temporal lobe epilepsy (TLE). In this study, we investigate whether integrity of the white matter network that interconnects this basal region to the distributed language network affects naming ability and risk for post-surgical naming decline.

METHODS

Patients with drug-resistant TLE were recruited from two epilepsy centers in a prospective longitudinal study. The pre-surgical dataset included 50 healthy controls, 47 left TLE (L-TLE), and 41 right TLE (R-TLE) patients. All participants completed pre-surgical T1- and diffusion-weighted MRI (dMRI), as well as neuropsychological tests of auditory and visual naming. Nineteen L-TLE and 18 R-TLE patients underwent anterior temporal lobectomy (ATL) and also completed post-surgical neuropsychological testing. Pre-surgical fractional anisotropy (FA) of the white matter directly beneath the fusiform neocortex (i.e., superficial white matter; SWM) and of deep white matter tracts with connections to the basal-temporal cortex [inferior longitudinal fasciculus (ILF) and inferior frontal occipital fasciculus (IFOF)] was calculated. Clinical variables, hippocampal volume, and FA of each white matter tract or region were examined in linear regressions with naming scores, or change in naming scores, as the primary outcomes.

RESULTS

Pre-surgically, higher FA in the bilateral ILF, bilateral IFOF, and left fusiform SWM was associated with better visual and auditory naming scores (all ps < 0.05 with FDR correction). In L-TLE, higher pre-surgical FA was also associated with less naming decline post-surgically, but results varied across tracts. When including only patients with typical language dominance, only integrity of the right fusiform SWM was associated with less visual naming decline (p = .0018).

DISCUSSION

Although a broad network of white matter network matter may contribute to naming ability pre-surgically, the reserve capacity of the contralateral (right) fusiform SWM may be important for mitigating visual naming decline following ATL in L-TLE. This shows that the study of the structural network interconnecting the basal-temporal region to the wider language network has implications for understanding both pre- and post-surgical naming in TLE.

Missing links: The functional unification of language and memory (L∪M)

The field of neurocognition is currently undergoing a significant change of perspective. Traditional neurocognitive models evolved into an integrative and dynamic vision of cognitive functioning. Dynamic integration assumes an interaction between cognitive domains traditionally considered to be distinct. Language and declarative memory are regarded as separate functions supported by different neural systems. However, they also share anatomical structures (notably, the inferior frontal gyrus, the supplementary motor area, the superior and middle temporal gyrus, and the hippocampal complex) and cognitive processes (such as semantic and working memory) that merge to endorse our quintessential daily lives. We propose a new model, "L∪M" (i.e., Language/union/Memory), that considers these two functions interactively. We fractionated language and declarative memory into three fundamental dimensions or systems ("Receiver-Transmitter", "Controller-Manager" and "Transformer-Associative" Systems), that communicate reciprocally. We formalized their interactions at the brain level with a connectivity-based approach. This new taxonomy overcomes the modular view of cognitive functioning and reconciles functional specialization with plasticity in neurological disorders.

No consolidation without representation: Correspondence between neural and psychological representations in recent and remote memory

Memory systems consolidation is often conceived as the linear, time-dependent, neurobiological shift of memory from hippocampal-cortical to cortico-cortical dependency. We argue that contrary to this unidirectional view of memory reorganization, information about events may be retained in multiple forms (e.g., event-specific sensory-near episodic memory, event-specific gist information, event-general schematic information, or abstract semantic memory). These representations can all form at the time of the event and may continue to coexist for long durations. Their relative strength, composition, and dominance of expression change with time and experience, with task demands, and through their dynamic interaction with one another. These different psychological mnemonic representations depend on distinct functional and structural neurobiological substrates such that there is a neural-psychological representation correspondence (NPRC) among them. We discuss how the dynamics of psychological memory representations are reflected in multiple levels of neurobiological markers and their interactions. By this view, there are only variations of synaptic consolidation and memory dynamics without assuming a distinct systems consolidation process.